diff --git a/func-import.go b/func-import.go
index 81b5a44..c73eb3d 100644
--- a/func-import.go
+++ b/func-import.go
@@ -30,7 +30,7 @@ func importGeneral(ctx ExprContext, name string, args []any) (result any, err er
 
 	dirList = addEnvImportDirs(dirList)
 	dirList = addPresetImportDirs(ctx, dirList)
-	result, err = doImport(ctx, name, dirList, NewFlatArrayIterator(args))
+	result, err = doImport(ctx, name, dirList, NewArrayIterator(args))
 	return
 }
 
diff --git a/func-math.go b/func-math.go
index 08a869d..566d4b2 100644
--- a/func-math.go
+++ b/func-math.go
@@ -37,8 +37,8 @@ func doAdd(ctx ExprContext, name string, it Iterator) (result any, err error) {
 			if err = checkNumberParamExpected(name, v, it.Index()); err != nil {
 				break
 			}
-			if array, ok := v.(*ListType); ok {
-				if v, err = doAdd(ctx, name, NewFlatArrayIterator(*array)); err != nil {
+			if list, ok := v.(*ListType); ok {
+				if v, err = doAdd(ctx, name, NewListIterator(list)); err != nil {
 					break
 				}
 			}
@@ -87,7 +87,7 @@ func doAdd(ctx ExprContext, name string, it Iterator) (result any, err error) {
 }
 
 func addFunc(ctx ExprContext, name string, args []any) (result any, err error) {
-	result, err = doAdd(ctx, name, NewFlatArrayIterator(args))
+	result, err = doAdd(ctx, name, NewArrayIterator(args))
 	return
 }
 
@@ -113,8 +113,8 @@ func doMul(ctx ExprContext, name string, it Iterator) (result any, err error) {
 				break
 			}
 
-			if array, ok := v.(*ListType); ok {
-				if v, err = doMul(ctx, name, NewFlatArrayIterator(*array)); err != nil {
+			if list, ok := v.(*ListType); ok {
+				if v, err = doMul(ctx, name, NewListIterator(list)); err != nil {
 					break
 				}
 			}
@@ -163,7 +163,7 @@ func doMul(ctx ExprContext, name string, it Iterator) (result any, err error) {
 }
 
 func mulFunc(ctx ExprContext, name string, args []any) (result any, err error) {
-	result, err = doMul(ctx, name, NewFlatArrayIterator(args))
+	result, err = doMul(ctx, name, NewArrayIterator(args))
 	return
 }
 
diff --git a/func-string.go b/func-string.go
index 45291d0..27d57f0 100644
--- a/func-string.go
+++ b/func-string.go
@@ -39,15 +39,15 @@ func joinStrFunc(ctx ExprContext, name string, args []any) (result any, err erro
 		if len(args) == 1 {
 			result = ""
 		} else if len(args) == 2 {
-			if ls, ok := args[1].([]any); ok {
-				result, err = doJoinStr(name, sep, NewFlatArrayIterator(ls))
+			if ls, ok := args[1].(*ListType); ok {
+				result, err = doJoinStr(name, sep, NewListIterator(ls))
 			} else if it, ok := args[1].(Iterator); ok {
 				result, err = doJoinStr(name, sep, it)
 			} else {
 				err = errInvalidParameterValue(name, paramParts, args[1])
 			}
 		} else {
-			result, err = doJoinStr(name, sep, NewFlatArrayIterator(args[1:]))
+			result, err = doJoinStr(name, sep, NewArrayIterator(args[1:]))
 		}
 	} else {
 		err = errWrongParamType(name, paramSeparator, typeString, args[0])
diff --git a/iter-list.go b/iter-list.go
index c9ac728..8cfefa2 100644
--- a/iter-list.go
+++ b/iter-list.go
@@ -6,33 +6,53 @@ package expr
 
 import "io"
 
-type FlatArrayIterator struct {
-	a     ListType
+type ListIterator struct {
+	a     *ListType
 	index int
 }
 
-func NewFlatArrayIterator(array ListType) *FlatArrayIterator {
-	return &FlatArrayIterator{a: array, index: 0}
+func NewListIterator(list *ListType) *ListIterator {
+	return &ListIterator{a: list, index: 0}
 }
 
-func (it *FlatArrayIterator) HasOperation(name string) bool {
+func NewArrayIterator(array []any) *ListIterator {
+	return &ListIterator{a: (*ListType)(&array), index: 0}
+}
+
+func NewAnyIterator(value any) (it *ListIterator) {
+	if value == nil {
+		it = NewArrayIterator([]any{})
+	} else if list, ok := value.(*ListType); ok {
+		it = NewListIterator(list)
+	} else if array, ok := value.([]any); ok {
+		it = NewArrayIterator(array)
+	} else if it1, ok := value.(*ListIterator); ok {
+		it = it1
+	} else {
+		it = NewArrayIterator([]any{value})
+	}
+	return
+}
+
+func (it *ListIterator) HasOperation(name string) bool {
 	return false
 }
 
-func (it *FlatArrayIterator) CallOperation(name string, args []any) (any, error) {
+func (it *ListIterator) CallOperation(name string, args []any) (any, error) {
 	return nil, errNoOperation(name)
 }
 
-func (it *FlatArrayIterator) Current() (item any, err error) {
-	if it.index >= 0 && it.index < len(it.a) {
-		item = it.a[it.index]
+func (it *ListIterator) Current() (item any, err error) {
+	a := *(it.a)
+	if it.index >= 0 && it.index < len(a) {
+		item = a[it.index]
 	} else {
 		err = io.EOF
 	}
 	return
 }
 
-func (it *FlatArrayIterator) Next() (item any, err error) {
+func (it *ListIterator) Next() (item any, err error) {
 	if item, err = it.Current(); err != io.EOF {
 		it.index++
 	}
@@ -45,6 +65,6 @@ func (it *FlatArrayIterator) Next() (item any, err error) {
 	return
 }
 
-func (it *FlatArrayIterator) Index() int {
+func (it *ListIterator) Index() int {
 	return it.index - 1
 }
diff --git a/operator-builtin.go b/operator-builtin.go
index 093177d..3c3eb65 100644
--- a/operator-builtin.go
+++ b/operator-builtin.go
@@ -4,6 +4,8 @@
 // operator-builtin.go
 package expr
 
+import "io"
+
 //-------- builtin term
 
 func newBuiltinTerm(tk *Token) (inst *term) {
@@ -18,17 +20,19 @@ func newBuiltinTerm(tk *Token) (inst *term) {
 
 func evalBuiltin(ctx ExprContext, self *term) (v any, err error) {
 	var childValue any
-	var it Iterator
 
 	if childValue, err = self.evalPrefix(ctx); err != nil {
 		return
 	}
 
 	count := 0
-	if isList(childValue) {
-		list, _ := childValue.(*ListType)
-		it := NewFlatArrayIterator(*list)
-		for moduleSpec, err1 := it.Next(); err1 == nil; moduleSpec, err1 = it.Next() {
+	if isString(childValue) {
+		module, _ := childValue.(string)
+		count, err = ImportInContextByGlobPattern(ctx, module)
+	} else {
+		var moduleSpec any
+		it := NewAnyIterator(childValue)
+		for moduleSpec, err = it.Next(); err == nil; moduleSpec, err = it.Next() {
 			if module, ok := moduleSpec.(string); ok {
 				if ImportInContext(ctx, module) {
 					count++
@@ -41,11 +45,9 @@ func evalBuiltin(ctx ExprContext, self *term) (v any, err error) {
 				break
 			}
 		}
-	} else if isString(childValue) {
-		module, _ := childValue.(string)
-		count, err = ImportInContextByGlobPattern(ctx, module)
-	} else {
-		err = self.errIncompatibleType(childValue)
+		if err == io.EOF {
+			err = nil
+		}
 	}
 	if err == nil {
 		v = count