When translating an EBNF grammar to a wisent grammar, I'd like to make intermediate rules like "foo_*" for "{foo}", "foo_+" for "foo {foo}", and "foo_?" for "[foo]". The former two works fine, but when i try the latter one, i get errors like "wisent-parse-nonterminals: Symbol `bar' is used, but is not defined as a token and has no rules", where "bar" has a rule using "foo_?".

According to http://www.gnu.org/software/emacs/manual/html_node/elisp/Symbol-Type.html, '?' requires no special punctuation.

What is the problem here?