LL&LR parser
http://web.cse.ohio-state.edu/software/2231/web-sw2/extras/slides/27.Recursive-Descent-Parsing.pdf
https://blog.csdn.net/qq_25104587/article/details/90577646
https://stackoverflow.com/questions/45079788/recursive-descent-parser-easy-to-get-explanation
https://stackoverflow.com/questions/5975741/what-is-the-difference-between-ll-and-lr-parsing
https://web.stanford.edu/class/archive/cs/cs143/cs143.1128/handouts/090%20Top-Down%20Parsing.pdf
https://stackoverflow.com/questions/16165352/why-cant-a-ll-grammar-be-left-recursive
https://en.wikipedia.org/wiki/Left_recursion
http://www.cs.nuim.ie/~jpower/Courses/Previous/parsing/new-main.html
http://www.cs.nuim.ie/~jpower/Courses/Previous/parsing/node1.html
http://blog.reverberate.org/2013/07/ll-and-lr-parsing-demystified.html
http://www.cs.nuim.ie/~jpower/Courses/Previous/parsing/node25.html
http://www.cs.nuim.ie/~jpower/Courses/Previous/parsing/node24.html
https://belyenochi.github.io/2018/09/06/parser_04/
http://fuis.me/2015/02/22/Parsing-Expressions-by-Recursive-Descent/
https://stackoverflow.com/questions/45079788/recursive-descent-parser-easy-to-get-explanation