my $grammar = q {
  startrule: test
    test:      "primary"(?) "key"
};

##</code><code>##

ERROR (line 4): Untranslatable item encountered: "?"
                (Hint: Did you misspell "?" or forget to comment it out?)

##</code><code>##

Parse::RecDescent: Treating "startrule:" as a rule declaration
    Parse::RecDescent: Treating "test" as a subrule match
    Parse::RecDescent: Treating "test:" as a rule declaration
    Parse::RecDescent: Treating ""primary"" as an interpolated literal
                       terminal
    Parse::RecDescent: Treating "( ? )" as an implicit subrule
    Parse::RecDescent: Treating "_alternation_1_of_production_1_of_rule_test
                       :" as a rule declaration

       ERROR (line 4): Untranslatable item encountered: "?"
                (Hint: Did you misspell "?" or forget to comment it out?)
    Parse::RecDescent: Treating "_alternation_1_of_production_1_of_rule_test"
                       as a subrule match
    Parse::RecDescent: Treating ""key"" as an interpolated literal terminal