diff --git a/src/main.js b/src/main.js
index 62a0030ff..1faaeefff 100644
--- a/src/main.js
+++ b/src/main.js
@@ -75,11 +75,13 @@ function extend (options) {
     var proto = ExtendedVM.prototype = Object.create(ParentVM.prototype)
     utils.defProtected(proto, 'constructor', ExtendedVM)
     // copy prototype props
-    if (options.props) {
-        utils.extend(proto, options.props, function (key) {
-            // do not overwrite the ancestor ViewModel prototype methods
-            return !(key in ViewModel.prototype)
-        })
+    var props = options.props
+    if (props) {
+        for (var key in props) {
+            if (!(key in ViewModel.prototype)) {
+                proto[key] = props[key]
+            }
+        }
     }
     // convert template to documentFragment
     if (options.template) {
diff --git a/src/utils.js b/src/utils.js
index 0604aee98..3ca14e573 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -3,6 +3,8 @@ var config    = require('./config'),
 
 module.exports = {
 
+    // global storage for user-registered
+    // vms, partials and transitions
     vms         : {},
     partials    : {},
     transitions : {},
@@ -21,22 +23,33 @@ module.exports = {
         })
     },
 
+    /*
+     *  Accurate type check
+     */
     typeOf: function (obj) {
         return toString.call(obj).slice(8, -1)
     },
 
-    extend: function (obj, ext, qualifier) {
+    /*
+     *  simple extend
+     */
+    extend: function (obj, ext) {
         for (var key in ext) {
-            if (qualifier && !qualifier(key)) continue
             obj[key] = ext[key]
         }
     },
 
+    /*
+     *  log for debugging
+     */
     log: function () {
         if (config.debug) console.log.apply(console, arguments)
         return this
     },
     
+    /*
+     *  warn for debugging
+     */
     warn: function() {
         if (config.debug) console.warn.apply(console, arguments)
         return this