Modification of javascript inheritance framework

I've modified John Resig simple javascript inheritance framework, by adding set function with setter functionality. It seems that it works. Is it written correctly? Can you see some undesired behavior? Framework:

(function(){
   var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/;
   this.Class = function(){};
   Class.extend = function(prop) {
     var _super = this.prototype;
    initializing = true;
    var prototype = new this();
    initializing = false;
    for (var name in prop) {
      prototype[name] = typeof prop[name] == "function" &&
        typeof _super[name] == "function" && fnTest.test(prop[name]) ?
        (function(name, fn){
          return function() {
            var tmp = this._super;
            this._super = _super[name];
            var ret = fn.apply(this, arguments);       
            // CHANGED HERE FROM this._super = tmp;
            // don't like some not used properties 
            if(tmp)this._super = tmp;
            // END OF CHANGE
            return ret;
          };
        })(name, prop[name]) :
        prop[name];
    }
    function Class() {
      if ( !initializing && this.init )
        this.init.apply(this, arguments);
    }
    Class.prototype = prototype;
    Class.prototype.constructor = Class;
    Class.extend = arguments.callee;

    // CHANGED HERE
    // Set method
    Class.prototype.set = function(attrs) {
      for (var attr in attrs) {
        //if exist setter function this.on_change{key of wrap: function(key of wrap, new value)
        if(this.on_change[attr])this.on_change[attr].call(this,attr,attrs[attr]);
        else this[attr] = attrs[attr];
      }
      return this;
    };
    if(!Class.prototype.on_change)Class.prototype.on_change={};
    // END OF CHANGE

    return Class;
  };
})();

Test code:

var Person = Class.extend({
  a: 15,
  b: 30,
  init: function(){
      //...
  },
  on_change:{
      b: function(oldKey, set){this[oldKey] = set*2}
  }
});
var p = new Person();
p.set({a:25,b:40});