Re: grammar for propositional logic

[John P. Hartmann]

Aha!

$undefined means that the lexer returned a character value that you
are not prepared for, that is, it is not in a rule or %token in your
grammar file.  Bison could have been a bit more cooperative by telling
you which character it is, but there you are.

Assuming the lexer is flex,  %option debug in your .l file will get
you a trace.  When you don't want to see the trace, you can set
yy_flex_debug=0 to turn it off without having to change the .l file.

   j.