extend = function(cls, prot) {
  for (var key in prot) {
    if (prot[key] instanceof Function) {
      if (cls.prototype[key] instanceof Function) {
        console.log(("Warning: Overwriting " + (cls.name) + ".prototype." + key));
      }
      cls.prototype[key] = prot[key];
    }
  }
}
@param {Class} cls
@param {Array} prot