> Calc>> lexer "4 * (7 + 1)"
> [Numeral 4,Mulop Times,LParen,Numeral 7,Addop Plus,Numeral 1,RParen]
> Calc>> expr (lexer "4711") :: Result Int
> Return 4711
> Calc>> expr (lexer "4 * (7 + 1) - 1") :: Result Int
> Return 31
> Calc>> expr (lexer "4 * (7 + 1 - 1") :: Result Int
> Fail "syntax error"