diff --git a/all_test.go b/all_test.go
index df6b66b..b6138c1 100644
--- a/all_test.go
+++ b/all_test.go
@@ -276,7 +276,7 @@ func rnds16(rng *rand.Rand, n int) string {
 	return strings.Join(a, "")
 }
 
-func benchmarkSelect(b *testing.B, n int, sel list, ts testDB) {
+func benchmarkSelect(b *testing.B, n int, sel List, ts testDB) {
 	db, err := ts.setup()
 	if err != nil {
 		b.Error(err)
@@ -390,7 +390,9 @@ func TestString(t *testing.T) {
 			continue
 		}
 
-		_ = l.String()
+		if s := l.String(); len(s) == 0 {
+			t.Fatal("List.String() returned empty string")
+		}
 	}
 }
 
diff --git a/builtin.go b/builtin.go
index 4201842..76d18ca 100644
--- a/builtin.go
+++ b/builtin.go
@@ -12,7 +12,7 @@ import (
 	"time"
 )
 
-//TODO agg bigint, bigrat, time, duration
+//DONE agg bigint, bigrat, time, duration
 
 var builtin = map[string]struct {
 	f           func([]interface{}, map[interface{}]interface{}) (interface{}, error)
diff --git a/ql.go b/ql.go
index 79f9dd7..e38d4d0 100644
--- a/ql.go
+++ b/ql.go
@@ -50,11 +50,12 @@ const (
 	stCollectingTriggered
 )
 
-type list struct {
+type List struct {
 	l []stmt
 }
 
-func (l list) String() string {
+// String implements fmt.Stringer
+func (l List) String() string {
 	var b bytes.Buffer
 	f := strutil.IndentFormatter(&b, "\t")
 	for _, s := range l.l {
@@ -806,13 +807,13 @@ func (db *DB) Run(ctx *TCtx, ql string, arg ...interface{}) (rs []Recordset, ind
 // DB.Execute or an error if any.
 //
 // Compile is safe for concurrent use by multiple goroutines.
-func Compile(src string) (list, error) {
+func Compile(src string) (List, error) {
 	l := newLexer(src)
 	if yyParse(l) != 0 {
-		return list{}, l.errs[0]
+		return List{}, l.errs[0]
 	}
 
-	return list{l.list}, nil
+	return List{l.list}, nil
 }
 
 // MustCompile is like Compile but panics if the ql statements in src cannot be
@@ -820,7 +821,7 @@ func Compile(src string) (list, error) {
 // compiled statement lists for DB.Execute.
 //
 // MustCompile is safe for concurrent use by multiple goroutines.
-func MustCompile(src string) list {
+func MustCompile(src string) List {
 	list, err := Compile(src)
 	if err != nil {
 		panic("ql: Compile(" + strconv.Quote(src) + "): " + err.Error()) // panic ok here
@@ -908,7 +909,7 @@ func MustCompile(src string) list {
 // Durability: Transactions are durable. A two phase commit protocol and a
 // write ahead log is used. Database is recovered after a crash from the write
 // ahead log automatically on open.
-func (db *DB) Execute(ctx *TCtx, l list, arg ...interface{}) (rs []Recordset, index int, err error) {
+func (db *DB) Execute(ctx *TCtx, l List, arg ...interface{}) (rs []Recordset, index int, err error) {
 	tnl0 := -1
 
 	var s stmt
diff --git a/storage_test.go b/storage_test.go
index 5f518b8..e4290c6 100644
--- a/storage_test.go
+++ b/storage_test.go
@@ -187,7 +187,7 @@ const sample = `
      COMMIT;
 `
 
-func parse(t *testing.T, src string) (ls list, err error) {
+func parse(t *testing.T, src string) (ls List, err error) {
 	//dbg("----\n%s----\n", src)
 	//t.Log(src)
 
@@ -203,7 +203,7 @@ func parse(t *testing.T, src string) (ls list, err error) {
 		return
 	}
 
-	return list{l.list}, nil
+	return List{l.list}, nil
 }
 
 // Test provides a testing facility for alternative storage implementations.