I agree. Often, it's much easier to generate good diagnostic messages by adding failure paths to lexer or parser definitions. i.e. in the 01 case, add a rule fo

I agree. Often, it's much easier to generate good diagnostic messages by adding failure paths to lexer or parser definitions. i.e. in the 01 case, add a rule for numbers with leading zeroes to the lexer, which generates an error explaining that leading zeroes are prohibited.