let.js

   /**
   *  Returns an observable sequence that is the result of invoking the selector on the source sequence, without sharing subscriptions.
   *  This operator allows for a fluent style of writing queries that use the same sequence multiple times.
   *
   * @param {Function} selector Selector function which can use the source sequence as many times as needed, without sharing subscriptions to the source sequence.
   * @returns {Observable} An observable sequence that contains the elements of a sequence produced by multicasting the source sequence within a selector function.
   */
  observableProto.letBind = observableProto['let'] = function (func) {
    return func(this);
  };