code: function mixin(ctor, methods) {
  	    var keyCopier = function(key ) { ctor.prototype[key] = methods[key]; };
  	    Object.keys(methods).forEach(keyCopier);
  	    Object.getOwnPropertySymbols &&
  	      Object.getOwnPropertySymbols(methods).forEach(keyCopier);
  	    return ctor;
  	  }
docs: Contributes additional methods to a constructor