参考：
http://www.runoob.com/w3cnote/flex-grammar.html
https://www.jianshu.com/p/5856c4ae91f2
http://www.ruanyifeng.com/blog/2015/07/flex-grammar.html
https://www.cnblogs.com/xuyuntao/articles/6391728.html
原文：https://www.cnblogs.com/XJT2018/p/10642859.html