Contributes additional methods to a constructor
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;
	  }