Mootools奇怪的bug与jQuery冲突

请尝试运行以下代码:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script> b = jQuery.noConflict(true); </script>

<script src="http://ajax.googleapis.com/ajax/libs/mootools/1.2.3/mootools-yui-compressed.js"></script>
<script>
  (function($){  
    $.a = function(){
      var x = [];
      for (l in x){
        console.log(l);
      }
    };
  })(b);

  b.a();
</script>

即使x是空的,在 console.log