gistfile1.txt

use v6;

grammar Goo {
    token ident { ^ ')'? 'A'+ $ }
    token qwert { <ident> | 'yuiop' }
}

test('ident', |< AA )A >);
test('qwert', |< AA )A >);

sub test (Str $top, *@egs) {
    for @egs -> $eg {
        say (Goo.parse( $eg, :rule($top) ) ?? "OK  " !! "Fail")
          ~ " $eg  - $top";
    }
}

=begin Desc

The above contrived program prints:

    OK   AA  - ident
    OK   )A  - ident
    OK   AA  - qwert
    Fail )A  - qwert

Any of the following modifications allows all tests to pass:

    - Replace 'ident' with something else, like 'glorp'.

    - Remove the 'yuiop' alternation from <qwert>.

    - Replace the ')' prefix with something like 'x' (in both the rule
      and the sample data).

In each case, what's going on?

(It turns out I was unsuccessfully using an equally non-predefined
'ident' in a grammar I'm designing. It took me a couple of hours to
golf it down to this hopefully useful example.)

=end Desc