diff --git a/dist/xstream.js b/dist/xstream.js
index 721a345..40f33d9 100644
--- a/dist/xstream.js
+++ b/dist/xstream.js
@@ -940,6 +940,9 @@ var Stream = (function () {
             }
         }
     };
+    Stream.prototype.ctor = function () {
+        return this instanceof MemoryStream ? MemoryStream : Stream;
+    };
     
     Stream.create = function (producer) {
         if (producer) {
@@ -1008,19 +1011,23 @@ var Stream = (function () {
         }
         return new Stream(new MergeProducer(streams));
     };
-    
-    Stream.prototype.map = function (project) {
+    Stream.prototype._map = function (project) {
         var p = this._prod;
+        var ctor = this.ctor();
         if (p instanceof FilterOperator) {
-            return new Stream(new FilterMapOperator(p.passes, project, p.ins));
+            return new ctor(new FilterMapOperator(p.passes, project, p.ins));
         }
         if (p instanceof FilterMapOperator) {
-            return new Stream(new FilterMapOperator(p.passes, compose2(project, p.project), p.ins));
+            return new ctor(new FilterMapOperator(p.passes, compose2(project, p.project), p.ins));
         }
         if (p instanceof MapOperator) {
-            return new Stream(new MapOperator(compose2(project, p.project), p.ins));
+            return new ctor(new MapOperator(compose2(project, p.project), p.ins));
         }
-        return new Stream(new MapOperator(project, this));
+        return new ctor(new MapOperator(project, this));
+    };
+    
+    Stream.prototype.map = function (project) {
+        return this._map(project);
     };
     
     Stream.prototype.mapTo = function (projectedValue) {
@@ -1039,7 +1046,7 @@ var Stream = (function () {
     };
     
     Stream.prototype.take = function (amount) {
-        return new Stream(new TakeOperator(amount, this));
+        return new (this.ctor())(new TakeOperator(amount, this));
     };
     
     Stream.prototype.drop = function (amount) {
@@ -1051,19 +1058,19 @@ var Stream = (function () {
     };
     
     Stream.prototype.startWith = function (initial) {
-        return new Stream(new StartWithOperator(this, initial));
+        return new MemoryStream(new StartWithOperator(this, initial));
     };
     
     Stream.prototype.endWhen = function (other) {
-        return new Stream(new EndWhenOperator(other, this));
+        return new (this.ctor())(new EndWhenOperator(other, this));
     };
     
     Stream.prototype.fold = function (accumulate, seed) {
-        return new Stream(new FoldOperator(accumulate, seed, this));
+        return new MemoryStream(new FoldOperator(accumulate, seed, this));
     };
     
     Stream.prototype.replaceError = function (replace) {
-        return new Stream(new ReplaceErrorOperator(replace, this));
+        return new (this.ctor())(new ReplaceErrorOperator(replace, this));
     };
     
     Stream.prototype.flatten = function () {
@@ -1090,7 +1097,7 @@ var Stream = (function () {
     };
     
     Stream.prototype.debug = function (labelOrSpy) {
-        return new Stream(new DebugOperator(labelOrSpy, this));
+        return new (this.ctor())(new DebugOperator(labelOrSpy, this));
     };
     
     Stream.prototype.shamefullySendNext = function (value) {
@@ -1134,6 +1141,9 @@ var MimicStream = (function (_super) {
     };
     
     MimicStream.prototype.imitate = function (other) {
+        if (other instanceof MemoryStream) {
+            throw new Error('bad');
+        }
         this._target = other;
     };
     return MimicStream;
@@ -1160,6 +1170,24 @@ var MemoryStream = (function (_super) {
         this._has = false;
         _super.prototype._x.call(this);
     };
+    MemoryStream.prototype.map = function (project) {
+        return this._map(project);
+    };
+    MemoryStream.prototype.mapTo = function (projectedValue) {
+        return _super.prototype.mapTo.call(this, projectedValue);
+    };
+    MemoryStream.prototype.take = function (amount) {
+        return _super.prototype.take.call(this, amount);
+    };
+    MemoryStream.prototype.endWhen = function (other) {
+        return _super.prototype.endWhen.call(this, other);
+    };
+    MemoryStream.prototype.replaceError = function (replace) {
+        return _super.prototype.replaceError.call(this, replace);
+    };
+    MemoryStream.prototype.debug = function (labelOrSpy) {
+        return _super.prototype.debug.call(this, labelOrSpy);
+    };
     return MemoryStream;
 }(Stream));
 exports.MemoryStream = MemoryStream;
diff --git a/dist/xstream.min.js b/dist/xstream.min.js
index 8b8e8aa..238e8bd 100644
--- a/dist/xstream.min.js
+++ b/dist/xstream.min.js
@@ -1 +1 @@
-(function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.xstream=f()}})(function(){var define,module,exports;return function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s}({1:[function(require,module,exports){"use strict";var __extends=this&&this.__extends||function(d,b){for(var p in b)if(b.hasOwnProperty(p))d[p]=b[p];function __(){this.constructor=d}d.prototype=b===null?Object.create(b):(__.prototype=b.prototype,new __)};var empty={};function noop(){}function copy(a){var l=a.length;var b=Array(l);for(var i=0;i<l;++i){b[i]=a[i]}return b}exports.emptyListener={_n:noop,_e:noop,_c:noop};function internalizeProducer(producer){producer._start=function _start(il){il.next=il._n;il.error=il._e;il.complete=il._c;this.start(il)};producer._stop=producer.stop}function invoke(f,args){switch(args.length){case 0:return f();case 1:return f(args[0]);case 2:return f(args[0],args[1]);case 3:return f(args[0],args[1],args[2]);case 4:return f(args[0],args[1],args[2],args[3]);case 5:return f(args[0],args[1],args[2],args[3],args[4]);default:return f.apply(void 0,args)}}function compose2(f1,f2){return function composedFn(arg){return f1(f2(arg))}}function and(f1,f2){return function andFn(t){return f1(t)&&f2(t)}}var CombineListener=function(){function CombineListener(i,p){this.i=i;this.p=p;p.ils.push(this)}CombineListener.prototype._n=function(t){var p=this.p,out=p.out;if(!out)return;if(p.up(t,this.i)){try{out._n(invoke(p.project,p.vals))}catch(e){out._e(e)}}};CombineListener.prototype._e=function(err){var out=this.p.out;if(!out)return;out._e(err)};CombineListener.prototype._c=function(){var p=this.p;if(!p.out)return;if(--p.ac===0){p.out._c()}};return CombineListener}();exports.CombineListener=CombineListener;var CombineProducer=function(){function CombineProducer(project,streams){this.project=project;this.streams=streams;this.type="combine";this.out=exports.emptyListener;this.ils=[];var n=this.ac=this.left=streams.length;var vals=this.vals=new Array(n);for(var i=0;i<n;i++){vals[i]=empty}}CombineProducer.prototype.up=function(t,i){var v=this.vals[i];var left=!this.left?0:v===empty?--this.left:this.left;this.vals[i]=t;return left===0};CombineProducer.prototype._start=function(out){this.out=out;var s=this.streams;var n=s.length;if(n===0)this.zero(out);else{for(var i=0;i<n;i++){s[i]._add(new CombineListener(i,this))}}};CombineProducer.prototype._stop=function(){var s=this.streams;var n=this.ac=this.left=s.length;var vals=this.vals=new Array(n);for(var i=0;i<n;i++){s[i]._remove(this.ils[i]);vals[i]=empty}this.out=null;this.ils=[]};CombineProducer.prototype.zero=function(out){try{out._n(this.project());out._c()}catch(e){out._e(e)}};return CombineProducer}();exports.CombineProducer=CombineProducer;var FromArrayProducer=function(){function FromArrayProducer(a){this.a=a;this.type="fromArray"}FromArrayProducer.prototype._start=function(out){var a=this.a;for(var i=0,l=a.length;i<l;i++){out._n(a[i])}out._c()};FromArrayProducer.prototype._stop=function(){};return FromArrayProducer}();exports.FromArrayProducer=FromArrayProducer;var FromPromiseProducer=function(){function FromPromiseProducer(p){this.p=p;this.type="fromPromise";this.on=false}FromPromiseProducer.prototype._start=function(out){var prod=this;this.on=true;this.p.then(function(v){if(prod.on){out._n(v);out._c()}},function(e){out._e(e)}).then(null,function(err){setTimeout(function(){throw err})})};FromPromiseProducer.prototype._stop=function(){this.on=false};return FromPromiseProducer}();exports.FromPromiseProducer=FromPromiseProducer;var MergeProducer=function(){function MergeProducer(streams){this.streams=streams;this.type="merge";this.out=exports.emptyListener;this.ac=streams.length}MergeProducer.prototype._start=function(out){this.out=out;var s=this.streams;var L=s.length;for(var i=0;i<L;i++){s[i]._add(this)}};MergeProducer.prototype._stop=function(){var s=this.streams;var L=s.length;for(var i=0;i<L;i++){s[i]._remove(this)}this.out=null;this.ac=L};MergeProducer.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};MergeProducer.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MergeProducer.prototype._c=function(){if(--this.ac===0){var u=this.out;if(!u)return;u._c()}};return MergeProducer}();exports.MergeProducer=MergeProducer;var PeriodicProducer=function(){function PeriodicProducer(period){this.period=period;this.type="periodic";this.intervalID=-1;this.i=0}PeriodicProducer.prototype._start=function(stream){var self=this;function intervalHandler(){stream._n(self.i++)}this.intervalID=setInterval(intervalHandler,this.period)};PeriodicProducer.prototype._stop=function(){if(this.intervalID!==-1)clearInterval(this.intervalID);this.intervalID=-1;this.i=0};return PeriodicProducer}();exports.PeriodicProducer=PeriodicProducer;var DebugOperator=function(){function DebugOperator(arg,ins){this.ins=ins;this.type="debug";this.out=null;this.s=null;this.l=null;if(typeof arg==="string"){this.l=arg}else{this.s=arg}}DebugOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};DebugOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};DebugOperator.prototype._n=function(t){var u=this.out;if(!u)return;var s=this.s,l=this.l;if(s){try{s(t)}catch(e){u._e(e)}}else if(l){console.log(l+":",t)}else{console.log(t)}u._n(t)};DebugOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};DebugOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return DebugOperator}();exports.DebugOperator=DebugOperator;var DropOperator=function(){function DropOperator(max,ins){this.max=max;this.ins=ins;this.type="drop";this.out=null;this.dropped=0}DropOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};DropOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.dropped=0};DropOperator.prototype._n=function(t){var u=this.out;if(!u)return;if(this.dropped++>=this.max)u._n(t)};DropOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};DropOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return DropOperator}();exports.DropOperator=DropOperator;var OtherIL=function(){function OtherIL(out,op){this.out=out;this.op=op}OtherIL.prototype._n=function(t){this.op.end()};OtherIL.prototype._e=function(err){this.out._e(err)};OtherIL.prototype._c=function(){this.op.end()};return OtherIL}();var EndWhenOperator=function(){function EndWhenOperator(o,ins){this.o=o;this.ins=ins;this.type="endWhen";this.out=null;this.oil=exports.emptyListener}EndWhenOperator.prototype._start=function(out){this.out=out;this.o._add(this.oil=new OtherIL(out,this));this.ins._add(this)};EndWhenOperator.prototype._stop=function(){this.ins._remove(this);this.o._remove(this.oil);this.out=null;this.oil=null};EndWhenOperator.prototype.end=function(){var u=this.out;if(!u)return;u._c()};EndWhenOperator.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};EndWhenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};EndWhenOperator.prototype._c=function(){this.end()};return EndWhenOperator}();exports.EndWhenOperator=EndWhenOperator;var FilterOperator=function(){function FilterOperator(passes,ins){this.passes=passes;this.ins=ins;this.type="filter";this.out=null}FilterOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};FilterOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};FilterOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{if(this.passes(t))u._n(t)}catch(e){u._e(e)}};FilterOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FilterOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return FilterOperator}();exports.FilterOperator=FilterOperator;var FlattenListener=function(){function FlattenListener(out,op){this.out=out;this.op=op}FlattenListener.prototype._n=function(t){this.out._n(t)};FlattenListener.prototype._e=function(err){this.out._e(err)};FlattenListener.prototype._c=function(){this.op.inner=null;this.op.less()};return FlattenListener}();var FlattenOperator=function(){function FlattenOperator(ins){this.ins=ins;this.type="flatten";this.inner=null;this.il=null;this.open=true;this.out=null}FlattenOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};FlattenOperator.prototype._stop=function(){this.ins._remove(this);this.inner=null;this.il=null;this.open=true;this.out=null};FlattenOperator.prototype.less=function(){var u=this.out;if(!u)return;if(!this.open&&!this.inner)u._c()};FlattenOperator.prototype._n=function(s){var u=this.out;if(!u)return;var _a=this,inner=_a.inner,il=_a.il;if(inner&&il)inner._remove(il);(this.inner=s)._add(this.il=new FlattenListener(u,this))};FlattenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FlattenOperator.prototype._c=function(){this.open=false;this.less()};return FlattenOperator}();exports.FlattenOperator=FlattenOperator;var FoldOperator=function(){function FoldOperator(f,seed,ins){this.f=f;this.seed=seed;this.ins=ins;this.type="fold";this.out=null;this.acc=seed}FoldOperator.prototype._start=function(out){this.out=out;out._n(this.acc);this.ins._add(this)};FoldOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.acc=this.seed};FoldOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{u._n(this.acc=this.f(this.acc,t))}catch(e){u._e(e)}};FoldOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FoldOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return FoldOperator}();exports.FoldOperator=FoldOperator;var LastOperator=function(){function LastOperator(ins){this.ins=ins;this.type="last";this.out=null;this.has=false;this.val=empty}LastOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};LastOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.has=false;this.val=empty};LastOperator.prototype._n=function(t){this.has=true;this.val=t};LastOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};LastOperator.prototype._c=function(){var u=this.out;if(!u)return;if(this.has){u._n(this.val);u._c()}else{u._e("TODO show proper error")}};return LastOperator}();exports.LastOperator=LastOperator;var MapFlattenInner=function(){function MapFlattenInner(out,op){this.out=out;this.op=op}MapFlattenInner.prototype._n=function(r){this.out._n(r)};MapFlattenInner.prototype._e=function(err){this.out._e(err)};MapFlattenInner.prototype._c=function(){this.op.inner=null;this.op.less()};return MapFlattenInner}();var MapFlattenOperator=function(){function MapFlattenOperator(mapOp){this.mapOp=mapOp;this.inner=null;this.il=null;this.open=true;this.out=null;this.type=mapOp.type+"+flatten";this.ins=mapOp.ins}MapFlattenOperator.prototype._start=function(out){this.out=out;this.mapOp.ins._add(this)};MapFlattenOperator.prototype._stop=function(){this.mapOp.ins._remove(this);this.inner=null;this.il=null;this.open=true;this.out=null};MapFlattenOperator.prototype.less=function(){if(!this.open&&!this.inner){var u=this.out;if(!u)return;u._c()}};MapFlattenOperator.prototype._n=function(v){var u=this.out;if(!u)return;var _a=this,inner=_a.inner,il=_a.il;if(inner&&il)inner._remove(il);try{(this.inner=this.mapOp.project(v))._add(this.il=new MapFlattenInner(u,this))}catch(e){u._e(e)}};MapFlattenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MapFlattenOperator.prototype._c=function(){this.open=false;this.less()};return MapFlattenOperator}();exports.MapFlattenOperator=MapFlattenOperator;var MapOperator=function(){function MapOperator(project,ins){this.project=project;this.ins=ins;this.type="map";this.out=null}MapOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};MapOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};MapOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{u._n(this.project(t))}catch(e){u._e(e)}};MapOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MapOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return MapOperator}();exports.MapOperator=MapOperator;var FilterMapOperator=function(_super){__extends(FilterMapOperator,_super);function FilterMapOperator(passes,project,ins){_super.call(this,project,ins);this.passes=passes;this.type="filter+map"}FilterMapOperator.prototype._n=function(v){if(this.passes(v)){_super.prototype._n.call(this,v)}};return FilterMapOperator}(MapOperator);exports.FilterMapOperator=FilterMapOperator;var ReplaceErrorOperator=function(){function ReplaceErrorOperator(fn,ins){this.fn=fn;this.ins=ins;this.type="replaceError";this.out=empty}ReplaceErrorOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};ReplaceErrorOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};ReplaceErrorOperator.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};ReplaceErrorOperator.prototype._e=function(err){var u=this.out;if(!u)return;try{this.ins._remove(this);(this.ins=this.fn(err))._add(this)}catch(e){u._e(e)}};ReplaceErrorOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return ReplaceErrorOperator}();exports.ReplaceErrorOperator=ReplaceErrorOperator;var StartWithOperator=function(){function StartWithOperator(ins,value){this.ins=ins;this.value=value;this.type="startWith";this.out=exports.emptyListener}StartWithOperator.prototype._start=function(out){this.out=out;this.out._n(this.value);this.ins._add(out)};StartWithOperator.prototype._stop=function(){this.ins._remove(this.out);this.out=null};return StartWithOperator}();exports.StartWithOperator=StartWithOperator;var TakeOperator=function(){function TakeOperator(max,ins){this.max=max;this.ins=ins;this.type="take";this.out=null;this.taken=0}TakeOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};TakeOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.taken=0};TakeOperator.prototype._n=function(t){var u=this.out;if(!u)return;if(this.taken++<this.max-1){u._n(t)}else{u._n(t);u._c();this._stop()}};TakeOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};TakeOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return TakeOperator}();exports.TakeOperator=TakeOperator;var Stream=function(){function Stream(producer){this._stopID=empty;this.combine=function combine(project){var streams=[];for(var _i=1;_i<arguments.length;_i++){streams[_i-1]=arguments[_i]}streams.unshift(this);return Stream.combine.apply(Stream,[project].concat(streams))};this._prod=producer;this._ils=[]}Stream.prototype._n=function(t){var a=this._ils;var L=a.length;if(L==1)a[0]._n(t);else{var b=copy(a);for(var i=0;i<L;i++)b[i]._n(t)}};Stream.prototype._e=function(err){var a=this._ils;var L=a.length;if(L==1)a[0]._e(err);else{var b=copy(a);for(var i=0;i<L;i++)b[i]._e(err)}this._x()};Stream.prototype._c=function(){var a=this._ils;var L=a.length;if(L==1)a[0]._c();else{var b=copy(a);for(var i=0;i<L;i++)b[i]._c()}this._x()};Stream.prototype._x=function(){if(this._ils.length===0)return;if(this._prod)this._prod._stop();this._ils=[]};Stream.prototype.addListener=function(listener){if(typeof listener.next!=="function"||typeof listener.error!=="function"||typeof listener.complete!=="function"){throw new Error("stream.addListener() requires all three next, error, "+"and complete functions.")}listener._n=listener.next;listener._e=listener.error;listener._c=listener.complete;this._add(listener)};Stream.prototype.removeListener=function(listener){this._remove(listener)};Stream.prototype._add=function(il){var a=this._ils;a.push(il);if(a.length===1){if(this._stopID!==empty){clearTimeout(this._stopID);this._stopID=empty}var p=this._prod;if(p)p._start(this)}};Stream.prototype._remove=function(il){var a=this._ils;var i=a.indexOf(il);if(i>-1){a.splice(i,1);var p_1=this._prod;if(p_1&&a.length<=0){this._stopID=setTimeout(function(){return p_1._stop()})}}};Stream.create=function(producer){if(producer){if(typeof producer.start!=="function"||typeof producer.stop!=="function"){throw new Error("producer requires both start and stop functions")}internalizeProducer(producer)}return new Stream(producer)};Stream.createWithMemory=function(producer){if(producer){internalizeProducer(producer)}return new MemoryStream(producer)};Stream.createMimic=function(){return new MimicStream};Stream.never=function(){return new Stream({_start:noop,_stop:noop})};Stream.empty=function(){return new Stream({_start:function(il){il._c()},_stop:noop})};Stream.throw=function(error){return new Stream({_start:function(il){il._e(error)},_stop:noop})};Stream.of=function(){var items=[];for(var _i=0;_i<arguments.length;_i++){items[_i-0]=arguments[_i]}return Stream.fromArray(items)};Stream.fromArray=function(array){return new Stream(new FromArrayProducer(array))};Stream.fromPromise=function(promise){return new Stream(new FromPromiseProducer(promise))};Stream.periodic=function(period){return new Stream(new PeriodicProducer(period))};Stream.merge=function(){var streams=[];for(var _i=0;_i<arguments.length;_i++){streams[_i-0]=arguments[_i]}return new Stream(new MergeProducer(streams))};Stream.prototype.map=function(project){var p=this._prod;if(p instanceof FilterOperator){return new Stream(new FilterMapOperator(p.passes,project,p.ins))}if(p instanceof FilterMapOperator){return new Stream(new FilterMapOperator(p.passes,compose2(project,p.project),p.ins))}if(p instanceof MapOperator){return new Stream(new MapOperator(compose2(project,p.project),p.ins))}return new Stream(new MapOperator(project,this))};Stream.prototype.mapTo=function(projectedValue){var s=this.map(function(){return projectedValue});var op=s._prod;op.type=op.type.replace("map","mapTo");return s};Stream.prototype.filter=function(passes){var p=this._prod;if(p instanceof FilterOperator){return new Stream(new FilterOperator(and(passes,p.passes),p.ins))}return new Stream(new FilterOperator(passes,this))};Stream.prototype.take=function(amount){return new Stream(new TakeOperator(amount,this))};Stream.prototype.drop=function(amount){return new Stream(new DropOperator(amount,this))};Stream.prototype.last=function(){return new Stream(new LastOperator(this))};Stream.prototype.startWith=function(initial){return new Stream(new StartWithOperator(this,initial))};Stream.prototype.endWhen=function(other){return new Stream(new EndWhenOperator(other,this))};Stream.prototype.fold=function(accumulate,seed){return new Stream(new FoldOperator(accumulate,seed,this))};Stream.prototype.replaceError=function(replace){return new Stream(new ReplaceErrorOperator(replace,this))};Stream.prototype.flatten=function(){var p=this._prod;return new Stream(p instanceof MapOperator&&!(p instanceof FilterMapOperator)?new MapFlattenOperator(p):new FlattenOperator(this))};Stream.prototype.merge=function(other){return Stream.merge(this,other)};Stream.prototype.compose=function(operator){return operator(this)};Stream.prototype.remember=function(){var _this=this;return new MemoryStream({_start:function(il){_this._prod._start(il)},_stop:function(){_this._prod._stop()}})};Stream.prototype.debug=function(labelOrSpy){return new Stream(new DebugOperator(labelOrSpy,this))};Stream.prototype.shamefullySendNext=function(value){this._n(value)};Stream.prototype.shamefullySendError=function(error){this._e(error)};Stream.prototype.shamefullySendComplete=function(){this._c()};Stream.combine=function combine(project){var streams=[];for(var _i=1;_i<arguments.length;_i++){streams[_i-1]=arguments[_i]}return new Stream(new CombineProducer(project,streams))};return Stream}();exports.Stream=Stream;var MimicStream=function(_super){__extends(MimicStream,_super);function MimicStream(){_super.call(this)}MimicStream.prototype._add=function(il){var t=this._target;if(!t)return;t._add(il)};MimicStream.prototype._remove=function(il){var t=this._target;if(!t)return;t._remove(il)};MimicStream.prototype.imitate=function(other){this._target=other};return MimicStream}(Stream);exports.MimicStream=MimicStream;var MemoryStream=function(_super){__extends(MemoryStream,_super);function MemoryStream(producer){_super.call(this,producer);this._has=false}MemoryStream.prototype._n=function(x){this._v=x;this._has=true;_super.prototype._n.call(this,x)};MemoryStream.prototype._add=function(il){if(this._has){il._n(this._v)}_super.prototype._add.call(this,il)};MemoryStream.prototype._x=function(){this._has=false;_super.prototype._x.call(this)};return MemoryStream}(Stream);exports.MemoryStream=MemoryStream;Object.defineProperty(exports,"__esModule",{value:true});exports.default=Stream},{}],2:[function(require,module,exports){"use strict";var core_1=require("./core");exports.Stream=core_1.Stream;exports.MemoryStream=core_1.MemoryStream;exports.MimicStream=core_1.MimicStream;Object.defineProperty(exports,"__esModule",{value:true});exports.default=core_1.Stream},{"./core":1}]},{},[2])(2)});
\ No newline at end of file
+(function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.xstream=f()}})(function(){var define,module,exports;return function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s}({1:[function(require,module,exports){"use strict";var __extends=this&&this.__extends||function(d,b){for(var p in b)if(b.hasOwnProperty(p))d[p]=b[p];function __(){this.constructor=d}d.prototype=b===null?Object.create(b):(__.prototype=b.prototype,new __)};var empty={};function noop(){}function copy(a){var l=a.length;var b=Array(l);for(var i=0;i<l;++i){b[i]=a[i]}return b}exports.emptyListener={_n:noop,_e:noop,_c:noop};function internalizeProducer(producer){producer._start=function _start(il){il.next=il._n;il.error=il._e;il.complete=il._c;this.start(il)};producer._stop=producer.stop}function invoke(f,args){switch(args.length){case 0:return f();case 1:return f(args[0]);case 2:return f(args[0],args[1]);case 3:return f(args[0],args[1],args[2]);case 4:return f(args[0],args[1],args[2],args[3]);case 5:return f(args[0],args[1],args[2],args[3],args[4]);default:return f.apply(void 0,args)}}function compose2(f1,f2){return function composedFn(arg){return f1(f2(arg))}}function and(f1,f2){return function andFn(t){return f1(t)&&f2(t)}}var CombineListener=function(){function CombineListener(i,p){this.i=i;this.p=p;p.ils.push(this)}CombineListener.prototype._n=function(t){var p=this.p,out=p.out;if(!out)return;if(p.up(t,this.i)){try{out._n(invoke(p.project,p.vals))}catch(e){out._e(e)}}};CombineListener.prototype._e=function(err){var out=this.p.out;if(!out)return;out._e(err)};CombineListener.prototype._c=function(){var p=this.p;if(!p.out)return;if(--p.ac===0){p.out._c()}};return CombineListener}();exports.CombineListener=CombineListener;var CombineProducer=function(){function CombineProducer(project,streams){this.project=project;this.streams=streams;this.type="combine";this.out=exports.emptyListener;this.ils=[];var n=this.ac=this.left=streams.length;var vals=this.vals=new Array(n);for(var i=0;i<n;i++){vals[i]=empty}}CombineProducer.prototype.up=function(t,i){var v=this.vals[i];var left=!this.left?0:v===empty?--this.left:this.left;this.vals[i]=t;return left===0};CombineProducer.prototype._start=function(out){this.out=out;var s=this.streams;var n=s.length;if(n===0)this.zero(out);else{for(var i=0;i<n;i++){s[i]._add(new CombineListener(i,this))}}};CombineProducer.prototype._stop=function(){var s=this.streams;var n=this.ac=this.left=s.length;var vals=this.vals=new Array(n);for(var i=0;i<n;i++){s[i]._remove(this.ils[i]);vals[i]=empty}this.out=null;this.ils=[]};CombineProducer.prototype.zero=function(out){try{out._n(this.project());out._c()}catch(e){out._e(e)}};return CombineProducer}();exports.CombineProducer=CombineProducer;var FromArrayProducer=function(){function FromArrayProducer(a){this.a=a;this.type="fromArray"}FromArrayProducer.prototype._start=function(out){var a=this.a;for(var i=0,l=a.length;i<l;i++){out._n(a[i])}out._c()};FromArrayProducer.prototype._stop=function(){};return FromArrayProducer}();exports.FromArrayProducer=FromArrayProducer;var FromPromiseProducer=function(){function FromPromiseProducer(p){this.p=p;this.type="fromPromise";this.on=false}FromPromiseProducer.prototype._start=function(out){var prod=this;this.on=true;this.p.then(function(v){if(prod.on){out._n(v);out._c()}},function(e){out._e(e)}).then(null,function(err){setTimeout(function(){throw err})})};FromPromiseProducer.prototype._stop=function(){this.on=false};return FromPromiseProducer}();exports.FromPromiseProducer=FromPromiseProducer;var MergeProducer=function(){function MergeProducer(streams){this.streams=streams;this.type="merge";this.out=exports.emptyListener;this.ac=streams.length}MergeProducer.prototype._start=function(out){this.out=out;var s=this.streams;var L=s.length;for(var i=0;i<L;i++){s[i]._add(this)}};MergeProducer.prototype._stop=function(){var s=this.streams;var L=s.length;for(var i=0;i<L;i++){s[i]._remove(this)}this.out=null;this.ac=L};MergeProducer.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};MergeProducer.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MergeProducer.prototype._c=function(){if(--this.ac===0){var u=this.out;if(!u)return;u._c()}};return MergeProducer}();exports.MergeProducer=MergeProducer;var PeriodicProducer=function(){function PeriodicProducer(period){this.period=period;this.type="periodic";this.intervalID=-1;this.i=0}PeriodicProducer.prototype._start=function(stream){var self=this;function intervalHandler(){stream._n(self.i++)}this.intervalID=setInterval(intervalHandler,this.period)};PeriodicProducer.prototype._stop=function(){if(this.intervalID!==-1)clearInterval(this.intervalID);this.intervalID=-1;this.i=0};return PeriodicProducer}();exports.PeriodicProducer=PeriodicProducer;var DebugOperator=function(){function DebugOperator(arg,ins){this.ins=ins;this.type="debug";this.out=null;this.s=null;this.l=null;if(typeof arg==="string"){this.l=arg}else{this.s=arg}}DebugOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};DebugOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};DebugOperator.prototype._n=function(t){var u=this.out;if(!u)return;var s=this.s,l=this.l;if(s){try{s(t)}catch(e){u._e(e)}}else if(l){console.log(l+":",t)}else{console.log(t)}u._n(t)};DebugOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};DebugOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return DebugOperator}();exports.DebugOperator=DebugOperator;var DropOperator=function(){function DropOperator(max,ins){this.max=max;this.ins=ins;this.type="drop";this.out=null;this.dropped=0}DropOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};DropOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.dropped=0};DropOperator.prototype._n=function(t){var u=this.out;if(!u)return;if(this.dropped++>=this.max)u._n(t)};DropOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};DropOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return DropOperator}();exports.DropOperator=DropOperator;var OtherIL=function(){function OtherIL(out,op){this.out=out;this.op=op}OtherIL.prototype._n=function(t){this.op.end()};OtherIL.prototype._e=function(err){this.out._e(err)};OtherIL.prototype._c=function(){this.op.end()};return OtherIL}();var EndWhenOperator=function(){function EndWhenOperator(o,ins){this.o=o;this.ins=ins;this.type="endWhen";this.out=null;this.oil=exports.emptyListener}EndWhenOperator.prototype._start=function(out){this.out=out;this.o._add(this.oil=new OtherIL(out,this));this.ins._add(this)};EndWhenOperator.prototype._stop=function(){this.ins._remove(this);this.o._remove(this.oil);this.out=null;this.oil=null};EndWhenOperator.prototype.end=function(){var u=this.out;if(!u)return;u._c()};EndWhenOperator.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};EndWhenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};EndWhenOperator.prototype._c=function(){this.end()};return EndWhenOperator}();exports.EndWhenOperator=EndWhenOperator;var FilterOperator=function(){function FilterOperator(passes,ins){this.passes=passes;this.ins=ins;this.type="filter";this.out=null}FilterOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};FilterOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};FilterOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{if(this.passes(t))u._n(t)}catch(e){u._e(e)}};FilterOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FilterOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return FilterOperator}();exports.FilterOperator=FilterOperator;var FlattenListener=function(){function FlattenListener(out,op){this.out=out;this.op=op}FlattenListener.prototype._n=function(t){this.out._n(t)};FlattenListener.prototype._e=function(err){this.out._e(err)};FlattenListener.prototype._c=function(){this.op.inner=null;this.op.less()};return FlattenListener}();var FlattenOperator=function(){function FlattenOperator(ins){this.ins=ins;this.type="flatten";this.inner=null;this.il=null;this.open=true;this.out=null}FlattenOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};FlattenOperator.prototype._stop=function(){this.ins._remove(this);this.inner=null;this.il=null;this.open=true;this.out=null};FlattenOperator.prototype.less=function(){var u=this.out;if(!u)return;if(!this.open&&!this.inner)u._c()};FlattenOperator.prototype._n=function(s){var u=this.out;if(!u)return;var _a=this,inner=_a.inner,il=_a.il;if(inner&&il)inner._remove(il);(this.inner=s)._add(this.il=new FlattenListener(u,this))};FlattenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FlattenOperator.prototype._c=function(){this.open=false;this.less()};return FlattenOperator}();exports.FlattenOperator=FlattenOperator;var FoldOperator=function(){function FoldOperator(f,seed,ins){this.f=f;this.seed=seed;this.ins=ins;this.type="fold";this.out=null;this.acc=seed}FoldOperator.prototype._start=function(out){this.out=out;out._n(this.acc);this.ins._add(this)};FoldOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.acc=this.seed};FoldOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{u._n(this.acc=this.f(this.acc,t))}catch(e){u._e(e)}};FoldOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};FoldOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return FoldOperator}();exports.FoldOperator=FoldOperator;var LastOperator=function(){function LastOperator(ins){this.ins=ins;this.type="last";this.out=null;this.has=false;this.val=empty}LastOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};LastOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.has=false;this.val=empty};LastOperator.prototype._n=function(t){this.has=true;this.val=t};LastOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};LastOperator.prototype._c=function(){var u=this.out;if(!u)return;if(this.has){u._n(this.val);u._c()}else{u._e("TODO show proper error")}};return LastOperator}();exports.LastOperator=LastOperator;var MapFlattenInner=function(){function MapFlattenInner(out,op){this.out=out;this.op=op}MapFlattenInner.prototype._n=function(r){this.out._n(r)};MapFlattenInner.prototype._e=function(err){this.out._e(err)};MapFlattenInner.prototype._c=function(){this.op.inner=null;this.op.less()};return MapFlattenInner}();var MapFlattenOperator=function(){function MapFlattenOperator(mapOp){this.mapOp=mapOp;this.inner=null;this.il=null;this.open=true;this.out=null;this.type=mapOp.type+"+flatten";this.ins=mapOp.ins}MapFlattenOperator.prototype._start=function(out){this.out=out;this.mapOp.ins._add(this)};MapFlattenOperator.prototype._stop=function(){this.mapOp.ins._remove(this);this.inner=null;this.il=null;this.open=true;this.out=null};MapFlattenOperator.prototype.less=function(){if(!this.open&&!this.inner){var u=this.out;if(!u)return;u._c()}};MapFlattenOperator.prototype._n=function(v){var u=this.out;if(!u)return;var _a=this,inner=_a.inner,il=_a.il;if(inner&&il)inner._remove(il);try{(this.inner=this.mapOp.project(v))._add(this.il=new MapFlattenInner(u,this))}catch(e){u._e(e)}};MapFlattenOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MapFlattenOperator.prototype._c=function(){this.open=false;this.less()};return MapFlattenOperator}();exports.MapFlattenOperator=MapFlattenOperator;var MapOperator=function(){function MapOperator(project,ins){this.project=project;this.ins=ins;this.type="map";this.out=null}MapOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};MapOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};MapOperator.prototype._n=function(t){var u=this.out;if(!u)return;try{u._n(this.project(t))}catch(e){u._e(e)}};MapOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};MapOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return MapOperator}();exports.MapOperator=MapOperator;var FilterMapOperator=function(_super){__extends(FilterMapOperator,_super);function FilterMapOperator(passes,project,ins){_super.call(this,project,ins);this.passes=passes;this.type="filter+map"}FilterMapOperator.prototype._n=function(v){if(this.passes(v)){_super.prototype._n.call(this,v)}};return FilterMapOperator}(MapOperator);exports.FilterMapOperator=FilterMapOperator;var ReplaceErrorOperator=function(){function ReplaceErrorOperator(fn,ins){this.fn=fn;this.ins=ins;this.type="replaceError";this.out=empty}ReplaceErrorOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};ReplaceErrorOperator.prototype._stop=function(){this.ins._remove(this);this.out=null};ReplaceErrorOperator.prototype._n=function(t){var u=this.out;if(!u)return;u._n(t)};ReplaceErrorOperator.prototype._e=function(err){var u=this.out;if(!u)return;try{this.ins._remove(this);(this.ins=this.fn(err))._add(this)}catch(e){u._e(e)}};ReplaceErrorOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return ReplaceErrorOperator}();exports.ReplaceErrorOperator=ReplaceErrorOperator;var StartWithOperator=function(){function StartWithOperator(ins,value){this.ins=ins;this.value=value;this.type="startWith";this.out=exports.emptyListener}StartWithOperator.prototype._start=function(out){this.out=out;this.out._n(this.value);this.ins._add(out)};StartWithOperator.prototype._stop=function(){this.ins._remove(this.out);this.out=null};return StartWithOperator}();exports.StartWithOperator=StartWithOperator;var TakeOperator=function(){function TakeOperator(max,ins){this.max=max;this.ins=ins;this.type="take";this.out=null;this.taken=0}TakeOperator.prototype._start=function(out){this.out=out;this.ins._add(this)};TakeOperator.prototype._stop=function(){this.ins._remove(this);this.out=null;this.taken=0};TakeOperator.prototype._n=function(t){var u=this.out;if(!u)return;if(this.taken++<this.max-1){u._n(t)}else{u._n(t);u._c();this._stop()}};TakeOperator.prototype._e=function(err){var u=this.out;if(!u)return;u._e(err)};TakeOperator.prototype._c=function(){var u=this.out;if(!u)return;u._c()};return TakeOperator}();exports.TakeOperator=TakeOperator;var Stream=function(){function Stream(producer){this._stopID=empty;this.combine=function combine(project){var streams=[];for(var _i=1;_i<arguments.length;_i++){streams[_i-1]=arguments[_i]}streams.unshift(this);return Stream.combine.apply(Stream,[project].concat(streams))};this._prod=producer;this._ils=[]}Stream.prototype._n=function(t){var a=this._ils;var L=a.length;if(L==1)a[0]._n(t);else{var b=copy(a);for(var i=0;i<L;i++)b[i]._n(t)}};Stream.prototype._e=function(err){var a=this._ils;var L=a.length;if(L==1)a[0]._e(err);else{var b=copy(a);for(var i=0;i<L;i++)b[i]._e(err)}this._x()};Stream.prototype._c=function(){var a=this._ils;var L=a.length;if(L==1)a[0]._c();else{var b=copy(a);for(var i=0;i<L;i++)b[i]._c()}this._x()};Stream.prototype._x=function(){if(this._ils.length===0)return;if(this._prod)this._prod._stop();this._ils=[]};Stream.prototype.addListener=function(listener){if(typeof listener.next!=="function"||typeof listener.error!=="function"||typeof listener.complete!=="function"){throw new Error("stream.addListener() requires all three next, error, "+"and complete functions.")}listener._n=listener.next;listener._e=listener.error;listener._c=listener.complete;this._add(listener)};Stream.prototype.removeListener=function(listener){this._remove(listener)};Stream.prototype._add=function(il){var a=this._ils;a.push(il);if(a.length===1){if(this._stopID!==empty){clearTimeout(this._stopID);this._stopID=empty}var p=this._prod;if(p)p._start(this)}};Stream.prototype._remove=function(il){var a=this._ils;var i=a.indexOf(il);if(i>-1){a.splice(i,1);var p_1=this._prod;if(p_1&&a.length<=0){this._stopID=setTimeout(function(){return p_1._stop()})}}};Stream.prototype.ctor=function(){return this instanceof MemoryStream?MemoryStream:Stream};Stream.create=function(producer){if(producer){if(typeof producer.start!=="function"||typeof producer.stop!=="function"){throw new Error("producer requires both start and stop functions")}internalizeProducer(producer)}return new Stream(producer)};Stream.createWithMemory=function(producer){if(producer){internalizeProducer(producer)}return new MemoryStream(producer)};Stream.createMimic=function(){return new MimicStream};Stream.never=function(){return new Stream({_start:noop,_stop:noop})};Stream.empty=function(){return new Stream({_start:function(il){il._c()},_stop:noop})};Stream.throw=function(error){return new Stream({_start:function(il){il._e(error)},_stop:noop})};Stream.of=function(){var items=[];for(var _i=0;_i<arguments.length;_i++){items[_i-0]=arguments[_i]}return Stream.fromArray(items)};Stream.fromArray=function(array){return new Stream(new FromArrayProducer(array))};Stream.fromPromise=function(promise){return new Stream(new FromPromiseProducer(promise))};Stream.periodic=function(period){return new Stream(new PeriodicProducer(period))};Stream.merge=function(){var streams=[];for(var _i=0;_i<arguments.length;_i++){streams[_i-0]=arguments[_i]}return new Stream(new MergeProducer(streams))};Stream.prototype._map=function(project){var p=this._prod;var ctor=this.ctor();if(p instanceof FilterOperator){return new ctor(new FilterMapOperator(p.passes,project,p.ins))}if(p instanceof FilterMapOperator){return new ctor(new FilterMapOperator(p.passes,compose2(project,p.project),p.ins))}if(p instanceof MapOperator){return new ctor(new MapOperator(compose2(project,p.project),p.ins))}return new ctor(new MapOperator(project,this))};Stream.prototype.map=function(project){return this._map(project)};Stream.prototype.mapTo=function(projectedValue){var s=this.map(function(){return projectedValue});var op=s._prod;op.type=op.type.replace("map","mapTo");return s};Stream.prototype.filter=function(passes){var p=this._prod;if(p instanceof FilterOperator){return new Stream(new FilterOperator(and(passes,p.passes),p.ins))}return new Stream(new FilterOperator(passes,this))};Stream.prototype.take=function(amount){return new(this.ctor())(new TakeOperator(amount,this))};Stream.prototype.drop=function(amount){return new Stream(new DropOperator(amount,this))};Stream.prototype.last=function(){return new Stream(new LastOperator(this))};Stream.prototype.startWith=function(initial){return new MemoryStream(new StartWithOperator(this,initial))};Stream.prototype.endWhen=function(other){return new(this.ctor())(new EndWhenOperator(other,this))};Stream.prototype.fold=function(accumulate,seed){return new MemoryStream(new FoldOperator(accumulate,seed,this))};Stream.prototype.replaceError=function(replace){return new(this.ctor())(new ReplaceErrorOperator(replace,this))};Stream.prototype.flatten=function(){var p=this._prod;return new Stream(p instanceof MapOperator&&!(p instanceof FilterMapOperator)?new MapFlattenOperator(p):new FlattenOperator(this))};Stream.prototype.merge=function(other){return Stream.merge(this,other)};Stream.prototype.compose=function(operator){return operator(this)};Stream.prototype.remember=function(){var _this=this;return new MemoryStream({_start:function(il){_this._prod._start(il)},_stop:function(){_this._prod._stop()}})};Stream.prototype.debug=function(labelOrSpy){return new(this.ctor())(new DebugOperator(labelOrSpy,this))};Stream.prototype.shamefullySendNext=function(value){this._n(value)};Stream.prototype.shamefullySendError=function(error){this._e(error)};Stream.prototype.shamefullySendComplete=function(){this._c()};Stream.combine=function combine(project){var streams=[];for(var _i=1;_i<arguments.length;_i++){streams[_i-1]=arguments[_i]}return new Stream(new CombineProducer(project,streams))};return Stream}();exports.Stream=Stream;var MimicStream=function(_super){__extends(MimicStream,_super);function MimicStream(){_super.call(this)}MimicStream.prototype._add=function(il){var t=this._target;if(!t)return;t._add(il)};MimicStream.prototype._remove=function(il){var t=this._target;if(!t)return;t._remove(il)};MimicStream.prototype.imitate=function(other){if(other instanceof MemoryStream){throw new Error("bad")}this._target=other};return MimicStream}(Stream);exports.MimicStream=MimicStream;var MemoryStream=function(_super){__extends(MemoryStream,_super);function MemoryStream(producer){_super.call(this,producer);this._has=false}MemoryStream.prototype._n=function(x){this._v=x;this._has=true;_super.prototype._n.call(this,x)};MemoryStream.prototype._add=function(il){if(this._has){il._n(this._v)}_super.prototype._add.call(this,il)};MemoryStream.prototype._x=function(){this._has=false;_super.prototype._x.call(this)};MemoryStream.prototype.map=function(project){return this._map(project)};MemoryStream.prototype.mapTo=function(projectedValue){return _super.prototype.mapTo.call(this,projectedValue)};MemoryStream.prototype.take=function(amount){return _super.prototype.take.call(this,amount)};MemoryStream.prototype.endWhen=function(other){return _super.prototype.endWhen.call(this,other)};MemoryStream.prototype.replaceError=function(replace){return _super.prototype.replaceError.call(this,replace)};MemoryStream.prototype.debug=function(labelOrSpy){return _super.prototype.debug.call(this,labelOrSpy)};return MemoryStream}(Stream);exports.MemoryStream=MemoryStream;Object.defineProperty(exports,"__esModule",{value:true});exports.default=Stream},{}],2:[function(require,module,exports){"use strict";var core_1=require("./core");exports.Stream=core_1.Stream;exports.MemoryStream=core_1.MemoryStream;exports.MimicStream=core_1.MimicStream;Object.defineProperty(exports,"__esModule",{value:true});exports.default=core_1.Stream},{"./core":1}]},{},[2])(2)});
\ No newline at end of file
diff --git a/markdown/footer.md b/markdown/footer.md
index 032d5f9..a709691 100644
--- a/markdown/footer.md
+++ b/markdown/footer.md
@@ -4,6 +4,12 @@ The operators and factories listed above are the core functions. `xstream` has p
 
 # FAQ
 
+**Q: Why does `imitate()` support a Stream but not a MemoryStream?**
+
+A: MemoryStreams are meant for representing "values over time" (your age), while Streams represent simply events (your birthdays). MemoryStreams are usually initialized with a value, and `imitate()` is meant for creating circular dependencies of streams. If we would attempt to imitate a MemoryStream in a circular dependency, we would either get a race condition (where the symptom would be "nothing happens") or an infinite cyclic emission of values.
+
+If you find yourself wanting to use `imitate()` with a MemoryStream, you should rework your code around `imitate()` to use a Stream instead. Look for the stream in the circular dependency that represents an event stream, and that would be a candidate for creating a MimicStream which then imitates the real event stream.
+
 **Q: What's the difference between xstream and RxJS?**
 
 A: Read this [blog post](http://staltz.com/why-we-built-xstream.html) on the topic.
diff --git a/src/core.ts b/src/core.ts
index a2cbfaf..55ad963 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -1046,6 +1046,10 @@ export class Stream<T> implements InternalListener<T> {
     }
   }
 
+  private ctor(): typeof Stream {
+    return this instanceof MemoryStream ? MemoryStream : Stream;
+  }
+
   /**
    * Creates a new Stream given a Producer.
    *
@@ -1295,46 +1299,51 @@ export class Stream<T> implements InternalListener<T> {
       return new Stream<R>(new CombineProducer<R>(project, streams));
     };
 
-  /**
-   * Transforms each event from the input Stream through a `project` function,
-   * to get a Stream that emits those transformed events.
-   *
-   * Marble diagram:
-   *
-   * ```text
-   * --1---3--5-----7------
-   *    map(i => i * 10)
-   * --10--30-50----70-----
-   * ```
-   *
-   * @param {Function} project A function of type `(t: T) => U` that takes event
-   * `t` of type `T` from the input Stream and produces an event of type `U`, to
-   * be emitted on the output Stream.
-   * @return {Stream}
-   */
-  map<U>(project: (t: T) => U): Stream<U> {
+  protected _map<U>(project: (t: T) => U): Stream<U> | MemoryStream<U> {
     const p = this._prod;
+    const ctor = this.ctor();
     if (p instanceof FilterOperator) {
-      return new Stream<U>(new FilterMapOperator(
+      return new ctor<U>(new FilterMapOperator(
         (<FilterOperator<T>> p).passes,
         project,
         (<FilterOperator<T>> p).ins
       ));
     }
     if (p instanceof FilterMapOperator) {
-      return new Stream<U>(new FilterMapOperator(
+      return new ctor<U>(new FilterMapOperator(
         (<FilterMapOperator<T, T>> p).passes,
         compose2(project, (<FilterMapOperator<T, T>> p).project),
         (<FilterMapOperator<T, T>> p).ins
       ));
     }
     if (p instanceof MapOperator) {
-      return new Stream<U>(new MapOperator(
+      return new ctor<U>(new MapOperator(
         compose2(project, (<MapOperator<T, T>> p).project),
         (<MapOperator<T, T>> p).ins
       ));
     }
-    return new Stream<U>(new MapOperator(project, this));
+    return new ctor<U>(new MapOperator(project, this));
+  }
+
+  /**
+   * Transforms each event from the input Stream through a `project` function,
+   * to get a Stream that emits those transformed events.
+   *
+   * Marble diagram:
+   *
+   * ```text
+   * --1---3--5-----7------
+   *    map(i => i * 10)
+   * --10--30-50----70-----
+   * ```
+   *
+   * @param {Function} project A function of type `(t: T) => U` that takes event
+   * `t` of type `T` from the input Stream and produces an event of type `U`, to
+   * be emitted on the output Stream.
+   * @return {Stream}
+   */
+  map<U>(project: (t: T) => U): Stream<U> {
+    return this._map(project);
   }
 
   /**
@@ -1408,7 +1417,7 @@ export class Stream<T> implements InternalListener<T> {
    * @return {Stream}
    */
   take(amount: number): Stream<T> {
-    return new Stream<T>(new TakeOperator(amount, this));
+    return new (this.ctor())<T>(new TakeOperator(amount, this));
   }
 
   /**
@@ -1465,8 +1474,8 @@ export class Stream<T> implements InternalListener<T> {
    * @param initial The value or event to prepend.
    * @return {Stream}
    */
-  startWith(initial: T): Stream<T> {
-    return new Stream<T>(new StartWithOperator(this, initial));
+  startWith(initial: T): MemoryStream<T> {
+    return new MemoryStream<T>(new StartWithOperator(this, initial));
   }
 
   /**
@@ -1489,7 +1498,7 @@ export class Stream<T> implements InternalListener<T> {
    * @return {Stream}
    */
   endWhen(other: Stream<any>): Stream<T> {
-    return new Stream<T>(new EndWhenOperator(other, this));
+    return new (this.ctor())<T>(new EndWhenOperator(other, this));
   }
 
   /**
@@ -1520,8 +1529,8 @@ export class Stream<T> implements InternalListener<T> {
    * @param seed The initial accumulated value, of type `R`.
    * @return {Stream}
    */
-  fold<R>(accumulate: (acc: R, t: T) => R, seed: R): Stream<R> {
-    return new Stream<R>(new FoldOperator(accumulate, seed, this));
+  fold<R>(accumulate: (acc: R, t: T) => R, seed: R): MemoryStream<R> {
+    return new MemoryStream<R>(new FoldOperator(accumulate, seed, this));
   }
 
   /**
@@ -1548,7 +1557,7 @@ export class Stream<T> implements InternalListener<T> {
    * @return {Stream}
    */
   replaceError(replace: (err: any) => Stream<T>): Stream<T> {
-    return new Stream<T>(new ReplaceErrorOperator(replace, this));
+    return new (this.ctor())<T>(new ReplaceErrorOperator(replace, this));
   }
 
   /**
@@ -1700,7 +1709,7 @@ export class Stream<T> implements InternalListener<T> {
    * @return {Stream}
    */
   debug(labelOrSpy?: string | ((t: T) => void)): Stream<T> {
-    return new Stream<T>(new DebugOperator(labelOrSpy, this));
+    return new (this.ctor())<T>(new DebugOperator(labelOrSpy, this));
   }
 
   /**
@@ -1770,9 +1779,15 @@ export class MimicStream<T> extends Stream<T> {
    * the current stream, making it re-emit whatever events are emitted by the
    * given `other` stream.
    *
-   * @param {Stream} other The stream to imitate on the current one.
+   * @param {Stream} other The stream to imitate on the current one. Must not be
+   * a MemoryStream.
    */
   imitate(other: Stream<T>): void {
+    if (other instanceof MemoryStream) {
+      throw new Error('A MemoryStream was given to imitate(), but it only ' +
+      'supports a Stream. Read more about this restriction here: ' +
+      'https://github.com/staltz/xstream#faq');
+    }
     this._target = other;
   }
 }
@@ -1799,6 +1814,30 @@ export class MemoryStream<T> extends Stream<T> {
     this._has = false;
     super._x();
   }
+
+  map<U>(project: (t: T) => U): MemoryStream<U> {
+    return <MemoryStream<U>> this._map(project);
+  }
+
+  mapTo<U>(projectedValue: U): MemoryStream<U> {
+    return <MemoryStream<U>> super.mapTo(projectedValue);
+  }
+
+  take(amount: number): MemoryStream<T> {
+    return <MemoryStream<T>> super.take(amount);
+  }
+
+  endWhen(other: Stream<any>): MemoryStream<T> {
+    return <MemoryStream<T>> super.endWhen(other);
+  }
+
+  replaceError(replace: (err: any) => Stream<T>): MemoryStream<T> {
+    return <MemoryStream<T>> super.replaceError(replace);
+  }
+
+  debug(labelOrSpy?: string | ((t: T) => void)): MemoryStream<T> {
+    return <MemoryStream<T>> super.debug(labelOrSpy);
+  }
 }
 
 export default Stream;
diff --git a/tests/extra/concat.ts b/tests/extra/concat.ts
index bf6dcf4..a64fd90 100644
--- a/tests/extra/concat.ts
+++ b/tests/extra/concat.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import concat from '../../src/extra/concat';
 import * as assert from 'assert';
diff --git a/tests/extra/debounce.ts b/tests/extra/debounce.ts
index 4592f25..093dcca 100644
--- a/tests/extra/debounce.ts
+++ b/tests/extra/debounce.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Listener, Producer} from '../../src/index';
 import debounce from '../../src/extra/debounce';
 import * as assert from 'assert';
diff --git a/tests/extra/delay.ts b/tests/extra/delay.ts
index c2afc8f..f60f5ba 100644
--- a/tests/extra/delay.ts
+++ b/tests/extra/delay.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import delay from '../../src/extra/delay';
 import * as assert from 'assert';
diff --git a/tests/extra/dropRepeats.ts b/tests/extra/dropRepeats.ts
index 53283b8..77172ad 100644
--- a/tests/extra/dropRepeats.ts
+++ b/tests/extra/dropRepeats.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Stream} from '../../src/index';
 import dropRepeats from '../../src/extra/dropRepeats';
 import * as assert from 'assert';
diff --git a/tests/extra/dropUntil.ts b/tests/extra/dropUntil.ts
index 43ea387..6bc1845 100644
--- a/tests/extra/dropUntil.ts
+++ b/tests/extra/dropUntil.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import dropUntil from '../../src/extra/dropUntil';
 import delay from '../../src/extra/delay';
diff --git a/tests/extra/flattenConcurrently.ts b/tests/extra/flattenConcurrently.ts
index deeada5..e318ecc 100644
--- a/tests/extra/flattenConcurrently.ts
+++ b/tests/extra/flattenConcurrently.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Stream, Listener} from '../../src/index';
 import flattenConcurrently from '../../src/extra/flattenConcurrently';
 import * as assert from 'assert';
diff --git a/tests/extra/flattenSequentially.ts b/tests/extra/flattenSequentially.ts
index 62fed64..f4b65d0 100644
--- a/tests/extra/flattenSequentially.ts
+++ b/tests/extra/flattenSequentially.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import flattenSequentially from '../../src/extra/flattenSequentially';
 import * as assert from 'assert';
diff --git a/tests/extra/fromDiagram.ts b/tests/extra/fromDiagram.ts
index fffbf93..fb8aeec 100644
--- a/tests/extra/fromDiagram.ts
+++ b/tests/extra/fromDiagram.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import fromDiagram from '../../src/extra/fromDiagram';
 import * as assert from 'assert';
diff --git a/tests/extra/fromEvent.ts b/tests/extra/fromEvent.ts
index f88b606..7f57f40 100644
--- a/tests/extra/fromEvent.ts
+++ b/tests/extra/fromEvent.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import fromEvent from '../../src/extra/fromEvent';
 import * as assert from 'assert';
diff --git a/tests/extra/pairwise.ts b/tests/extra/pairwise.ts
index 6a6a40b..1422197 100644
--- a/tests/extra/pairwise.ts
+++ b/tests/extra/pairwise.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import pairwise from '../../src/extra/pairwise';
 import * as assert from 'assert';
diff --git a/tests/extra/split.ts b/tests/extra/split.ts
index 2f7232b..0d07fa9 100644
--- a/tests/extra/split.ts
+++ b/tests/extra/split.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Stream} from '../../src/index';
 import split from '../../src/extra/split';
 import concat from '../../src/extra/concat';
diff --git a/tests/factory/combine.ts b/tests/factory/combine.ts
index 724dd71..49a841b 100644
--- a/tests/factory/combine.ts
+++ b/tests/factory/combine.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Stream} from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/factory/empty.ts b/tests/factory/empty.ts
index 49098a2..5046d58 100644
--- a/tests/factory/empty.ts
+++ b/tests/factory/empty.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 
 describe('xs.empty()', function() {
diff --git a/tests/factory/fromArray.ts b/tests/factory/fromArray.ts
index 95bea90..ea9c737 100644
--- a/tests/factory/fromArray.ts
+++ b/tests/factory/fromArray.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/factory/fromPromise.ts b/tests/factory/fromPromise.ts
index 8499153..2e7298d 100644
--- a/tests/factory/fromPromise.ts
+++ b/tests/factory/fromPromise.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import {Promise} from 'es6-promise';
 import xs from '../../src/index';
 import * as assert from 'assert';
diff --git a/tests/factory/merge.ts b/tests/factory/merge.ts
index b0e9071..2fb430b 100644
--- a/tests/factory/merge.ts
+++ b/tests/factory/merge.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/factory/never.ts b/tests/factory/never.ts
index ec3b50a..8b233c3 100644
--- a/tests/factory/never.ts
+++ b/tests/factory/never.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 
 describe('xs.never()', () => {
diff --git a/tests/factory/of.ts b/tests/factory/of.ts
index c89031d..e819100 100644
--- a/tests/factory/of.ts
+++ b/tests/factory/of.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/factory/throw.ts b/tests/factory/throw.ts
index 0733104..c652a09 100644
--- a/tests/factory/throw.ts
+++ b/tests/factory/throw.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/memoryStream.ts b/tests/memoryStream.ts
index 80d47af..38ae6fc 100644
--- a/tests/memoryStream.ts
+++ b/tests/memoryStream.ts
@@ -1,3 +1,5 @@
+/// <reference path="../typings/globals/mocha/index.d.ts" />
+/// <reference path="../typings/globals/node/index.d.ts" />
 import xs, {Producer, Listener, Stream} from '../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/mimicStream.ts b/tests/mimicStream.ts
index 24e29ce..723db81 100644
--- a/tests/mimicStream.ts
+++ b/tests/mimicStream.ts
@@ -1,3 +1,5 @@
+/// <reference path="../typings/globals/mocha/index.d.ts" />
+/// <reference path="../typings/globals/node/index.d.ts" />
 import xs, {Producer, Listener, Stream, MimicStream} from '../src/index';
 import delay from '../src/extra/delay';
 import * as assert from 'assert';
diff --git a/tests/operator/combine.ts b/tests/operator/combine.ts
index 9d53b1f..324d54c 100644
--- a/tests/operator/combine.ts
+++ b/tests/operator/combine.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.combine', () => {
@@ -54,4 +56,20 @@ describe('Stream.prototype.combine', () => {
       },
     });
   });
+
+  it('should return a Stream when combining a MemoryStream with a Stream', (done) => {
+    const input1 = xs.periodic(50).take(4).remember();
+    const input2 = xs.periodic(80).take(3);
+    const stream: Stream<number> = input1.combine((x, y) => x + y, input2);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a Stream when combining a MemoryStream with a MemoryStream', (done) => {
+    const input1 = xs.periodic(50).take(4).remember();
+    const input2 = xs.periodic(80).take(3).remember();
+    const stream: Stream<number> = input1.combine((x, y) => x + y, input2);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
 });
diff --git a/tests/operator/debug.ts b/tests/operator/debug.ts
index d798945..46e65d9 100644
--- a/tests/operator/debug.ts
+++ b/tests/operator/debug.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.debug', () => {
@@ -21,6 +23,22 @@ describe('Stream.prototype.debug', () => {
     stream.addListener(listener);
   });
 
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.debug('stream');
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.debug('stream');
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
+
   it('should propagate user mistakes in spy as errors', (done) => {
     const source = xs.periodic(30).take(1);
     const stream = source.debug(
diff --git a/tests/operator/drop.ts b/tests/operator/drop.ts
index 000ea26..1cdd7ec 100644
--- a/tests/operator/drop.ts
+++ b/tests/operator/drop.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.drop', () => {
@@ -18,4 +20,20 @@ describe('Stream.prototype.drop', () => {
     };
     stream.addListener(listener);
   });
+
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.drop(1);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a Stream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: Stream<number> = input.drop(1);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
 });
diff --git a/tests/operator/endWhen.ts b/tests/operator/endWhen.ts
index 7bb9270..45abf16 100644
--- a/tests/operator/endWhen.ts
+++ b/tests/operator/endWhen.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import delay from '../../src/extra/delay';
 import * as assert from 'assert';
 
@@ -38,4 +40,20 @@ describe('Stream.prototype.endWhen', () => {
       },
     });
   });
+
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.endWhen(xs.never());
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.endWhen(xs.never());
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
 });
diff --git a/tests/operator/filter.ts b/tests/operator/filter.ts
index d190ccd..1e7019a 100644
--- a/tests/operator/filter.ts
+++ b/tests/operator/filter.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.filter', () => {
@@ -98,4 +100,20 @@ describe('Stream.prototype.filter', () => {
     assert.strictEqual(stream['_prod']['type'], 'filter+mapTo');
     done();
   });
+
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.filter(x => x % 2 === 0);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a Stream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: Stream<number> = input.filter(x => x % 2 === 0);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
 });
diff --git a/tests/operator/flatten.ts b/tests/operator/flatten.ts
index 6c52235..26c1eae 100644
--- a/tests/operator/flatten.ts
+++ b/tests/operator/flatten.ts
@@ -1,3 +1,5 @@
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
 import xs, {Stream, Listener} from '../../src/index';
 import * as assert from 'assert';
 
diff --git a/tests/operator/fold.ts b/tests/operator/fold.ts
index 4c5ca71..2b529dc 100644
--- a/tests/operator/fold.ts
+++ b/tests/operator/fold.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.fold', () => {
@@ -41,4 +43,20 @@ describe('Stream.prototype.fold', () => {
       },
     });
   });
+
+  it('should return a MemoryStream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: MemoryStream<number> = input.fold((acc, x) => acc + x, 0);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.fold((acc, x) => acc + x, 0);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();;
+  })
 });
diff --git a/tests/operator/imitate.ts b/tests/operator/imitate.ts
index 65691e0..96c6c20 100644
--- a/tests/operator/imitate.ts
+++ b/tests/operator/imitate.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('MimicStream.prototype.imitate', () => {
@@ -37,23 +39,13 @@ describe('MimicStream.prototype.imitate', () => {
     proxyStream.imitate(stream);
   });
 
-//   it('should link the given stream to the mimic stream', (done) => {
-//     const stream = xs.periodic(50).take(3);
-//     const proxyStream = xs.createMimic<number>();
-//     proxyStream.imitate(stream);
-
-//     const expected = [0, 1, 2];
-//     setTimeout(() => {
-//       proxyStream.addListener({
-//         next: (x: number) => {
-//           assert.equal(x, expected.shift());
-//         },
-//         error: (err: any) => done(err),
-//         complete: () => {
-//           assert.equal(expected.length, 0);
-//           done();
-//         },
-//       });
-//     }, 125);
-//   })
+  it('should throw an error when given a MemoryStream', (done) => {
+    const stream = xs.periodic(50).take(3).remember();
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    const proxyStream = xs.createMimic<number>();
+    assert.throws(() => {
+      proxyStream.imitate(stream);
+    });
+    done();
+  });
 });
diff --git a/tests/operator/last.ts b/tests/operator/last.ts
index 172858b..1cd096c 100644
--- a/tests/operator/last.ts
+++ b/tests/operator/last.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.last', () => {
@@ -18,4 +20,20 @@ describe('Stream.prototype.last', () => {
     };
     stream.addListener(listener);
   });
+
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of<number>(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.last();
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a Stream if input stream is a MemoryStream', (done) => {
+    const input = xs.of<number>(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: Stream<number> = input.last();
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
 });
diff --git a/tests/operator/map.ts b/tests/operator/map.ts
index f4bd9c6..fa875bf 100644
--- a/tests/operator/map.ts
+++ b/tests/operator/map.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.map', () => {
@@ -57,6 +59,22 @@ describe('Stream.prototype.map', () => {
     done();
   });
 
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.map(x => x * 10);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream = input.map(x => x * 10);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
+
   it('should clean up Operator producer when failed', (done) => {
     const stream = xs.of<any>('a', 'b', 3).map(i => i.toUpperCase());
     const expected = ['A', 'B'];
diff --git a/tests/operator/mapTo.ts b/tests/operator/mapTo.ts
index 04fc73a..10c2b3a 100644
--- a/tests/operator/mapTo.ts
+++ b/tests/operator/mapTo.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.mapTo', () => {
@@ -18,6 +20,22 @@ describe('Stream.prototype.mapTo', () => {
     };
     stream.addListener(listener);
   });
+  
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.mapTo(10);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.mapTo(10);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
 
   it('should have \'type\' metadata on the operator producer', (done) => {
     const stream = xs.periodic(100).mapTo(10);
diff --git a/tests/operator/merge.ts b/tests/operator/merge.ts
index 56850bf..711afd7 100644
--- a/tests/operator/merge.ts
+++ b/tests/operator/merge.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.merge', () => {
@@ -34,4 +36,20 @@ describe('Stream.prototype.merge', () => {
       },
     });
   });
+
+  it('should return a Stream when merging a MemoryStream with a Stream', (done) => {
+    const input1 = xs.periodic(50).take(4).remember();
+    const input2 = xs.periodic(80).take(3);
+    const stream: Stream<number> = input1.merge(input2);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a Stream when merging a MemoryStream with a MemoryStream', (done) => {
+    const input1 = xs.periodic(50).take(4).remember();
+    const input2 = xs.periodic(80).take(3).remember();
+    const stream: Stream<number> = input1.merge(input2);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
 });
diff --git a/tests/operator/remember.ts b/tests/operator/remember.ts
index c9ec83e..487d70f 100644
--- a/tests/operator/remember.ts
+++ b/tests/operator/remember.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 function noop() {};
 
@@ -91,4 +93,20 @@ describe('Stream.prototype.remember', () => {
       },
     });
   });
+
+  it('should return a MemoryStream if input stream is a Stream', (done) => {
+    const input = xs.of(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: MemoryStream<number> = input.remember();
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  })
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.createWithMemory<number>();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.remember();
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
 });
diff --git a/tests/operator/replaceError.ts b/tests/operator/replaceError.ts
index f85f343..e0d2877 100644
--- a/tests/operator/replaceError.ts
+++ b/tests/operator/replaceError.ts
@@ -1,4 +1,6 @@
-import xs, {Producer} from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream, Producer} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.replaceError', () => {
@@ -127,4 +129,20 @@ describe('Stream.prototype.replaceError', () => {
       },
     });
   });
+
+  it('should return a Stream if input stream is a Stream', (done) => {
+    const input = xs.of(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.replaceError(err => xs.never());
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.replaceError(err => xs.never());
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
 });
diff --git a/tests/operator/startWith.ts b/tests/operator/startWith.ts
index 690015d..2a98c51 100644
--- a/tests/operator/startWith.ts
+++ b/tests/operator/startWith.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.startWith', () => {
@@ -13,4 +15,10 @@ describe('Stream.prototype.startWith', () => {
       complete: done
     });
   });
+  
+  it('should return a MemoryStream', (done) => {
+    const stream = xs.of(100).startWith(1);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
 });
diff --git a/tests/operator/take.ts b/tests/operator/take.ts
index b00a040..e6a91cf 100644
--- a/tests/operator/take.ts
+++ b/tests/operator/take.ts
@@ -1,4 +1,6 @@
-import xs from '../../src/index';
+/// <reference path="../../typings/globals/mocha/index.d.ts" />
+/// <reference path="../../typings/globals/node/index.d.ts" />
+import xs, {Stream, MemoryStream} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.take', () => {
@@ -19,6 +21,22 @@ describe('Stream.prototype.take', () => {
     stream.addListener(listener);
   });
 
+  it('should return a Stream if input stream is a tream', (done) => {
+    const input = xs.of(1, 2, 3);
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: Stream<number> = input.take(2);
+    assert.strictEqual(stream instanceof Stream, true);
+    done();
+  });
+
+  it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
+    const input = xs.of(1, 2, 3).remember();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.take(2);
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    done();
+  });
+
   it('should not break sibling listeners when TakeOperator tears down', (done) => {
     const source = xs.periodic(50);
     const streamA = source.take(3);
diff --git a/tests/stream.ts b/tests/stream.ts
index 99e61d0..13a203c 100644
--- a/tests/stream.ts
+++ b/tests/stream.ts
@@ -1,3 +1,5 @@
+/// <reference path="../typings/globals/mocha/index.d.ts" />
+/// <reference path="../typings/globals/node/index.d.ts" />
 import xs, {Producer, Listener, Stream} from '../src/index';
 import * as assert from 'assert';