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

Parser
Ignored blank;

exps = exp*;

exp = {add:} exp '+' factor |
      {min:} exp '-' factor |
      {factor:} factor ;
factor = {mul:} factor '*' term |
         {term:} term ;
term = {int:} int |
       {par:} '(' exp ')' ;