classDef = function() {
        if (arguments[0] !== Clazz) { this.construct.apply(this, arguments); }
    }
Can be used to build up inheritances of classes.
@example
var MyClass = Clazz.extend({
  construct: function(myParam){
    // Do sth.
  }
});
var MySubClass = MyClass.extend({
  construct: function(myParam){
    // Use this to call constructor of super class
    arguments.callee.$.construct.apply(this, arguments);
    // Do sth.
  }
});
@param {Object} def The definition of the new class.