gistfile1.pl

grammar Picker {

   rule combination($rx, $narity) {
        :my %*PICKED;
        <choose($rx)> ** $narity
    }

    rule choose($rx) {
        $<choice>=$rx <?{
            if %*PICKED{ ~$<choice> } {
                warn("oops, trying to repick: " ~$<choice>);
                False;
            }
            else {
               %*PICKED{ ~$<choice> } = True;
            }
         }>
    }
}

grammar Colors is Picker {

    token color {[red|orange|yellow|green|blue|indigo|violet]<!ww>}

     rule one-or-more-colors {
        $<choices>=<combination( rx[ <color> ], 1..* )>
    }
    
    rule any-two-colors {
        $<choices>=<combination( rx[ <color> ], 2..2 )>
    }
}

say Colors.parse(<red blue green>, :rule<one-or-more-colors>);
say Colors.parse(<red blue green blue>, :rule<one-or-more-colors>);

say Colors.parse(<red>, :rule<any-two-colors>);
say Colors.parse(<red blue>, :rule<any-two-colors>);
say Colors.parse(<red blue green>, :rule<any-two-colors>);