Implemented loop iterators

src/core/Define.go

@@ -10,7 +10,7 @@ import (
 // Define defines a new variable.
 func (f *Function) Define(leaf *expression.Expression) (*scope.Variable, error) {
 	name := leaf.Token.Text(f.File.Bytes)
-	variable, _ := f.Identifier(name)
+	variable := f.VariableByName(name)
 
 	if variable != nil {
 		return nil, errors.New(&errors.VariableAlreadyExists{Name: name}, f.File, leaf.Token.Position)