typed callback

multi a(&callback:(:$before, *@params)) {
        say &callback(1,2,3), 'before';
}

multi a(&callback:(:$after, *@params)) {
        say 'after', &callback(4,5,6);
}

my $cb1 = sub(:$after, *@params) {
        return @params.join('-');
}

my $cb2 = sub(:$before, *@params) {
        return @params.join('+');
}

a($cb1);
a($cb2);