问为什么我在Javascript中的extend()方法不适用于3个或更多的对象？

EN

extend: function (Child, Parent) {
  var F = function () {};
  F.prototype = Parent.prototype;
  Child.prototype = new F();
  Child.prototype.constructor = Child;
  Child.superclass = Parent.prototype;
  this.mixin(Child, Parent);
},
/**

 * @param {type} dst
 * @param {type} src
 * @returns {undefined}
 */
mixin: function (dst, src) {
  var tobj = {};
  for (var x in src) {
    if ((typeof tobj[x] == "undefined") || (tobj[x] != src[x])) {
      dst[x] = src[x];
    }
  }
  if (document.all && !document.isOpera) {
    var p = src.toString;
    if (typeof p == "function" && p != dst.toString && p != tobj.toString &&
      p != "\nfunction toString() {\n    [native code]\n}\n") {
      dst.toString = src.toString;
    }
  }
}

oopUtility.extend(Numbering, netBuilder);
oopUtility.extend(NumberingMethodDefault, Numbering);

Numbering.superclass.constructor.call(this, arguments);
NumberingMethodDefault.superclass.constructor.call(this, arguments);

Uncaught TypeError: Object #<NumberingMethodDefault> has no method 'setNumber'

console.log(NumberingMethodDefault.superclass);
//and it was netBuilder, not Numbering! о_О