set： https://developer.mozilla.org/zh-CN/docs/Web/javaScript/Reference/Global_Objects/Set

map: https://developer.mozilla.org/zh-CN/docs/Web/Javascript/Reference/Global_Objects/Map

for-of和迭代器: http://www.csdn.net/article/2015-06-15/2824965-es6-in-depth-iterators-and-the-for-of-loop