// See Kevin's excellent tutorial:
// http://www.kevlindev.com/tutorials/javascript/inheritance/index.htm

function extend(subclass, superclass) {
  function Dummy(){}
  Dummy.prototype = superclass.prototype;
  subclass.prototype = new Dummy();
  subclass.prototype.constructor = subclass;
  subclass.superclass = superclass;
  subclass.superproto = superclass.prototype;
};