Grammar arithmetique;
Lexer
int = ('0'..'9')+;
blank = ' ' | #9 | #10 | #13;

Parser
Ignored blank;

exps = exp*;

exp = {add:} exp '+' exp |
      {min:} exp '-' exp |
      {mul:} exp '*' exp |
      {int:} int |
      {par:} '(' exp ')' ;
//alt1   Precedence
//alt1      Left mul;
//alt1      Left add, min;