Add more comments to make the code clear

[isabelafoth]

Could you comment the code to be more clear about what does what? I'm having trouble modifying it and using it for my own needs.

[pengbo-learn]

1. Tokenizer do lexical analysis, generate a sequence of tokens defined by regular expression.
2. Parser do syntax analysis, parse the token sequence into a Tree.
3. Interpreter traverse the tree and execute the math expressions, which should be motified to satisfy your own needs.

The total lines of code is less than 700, most of which is quite straight forward.
If you could make your question more specific, I would be glad to help.