sub gen_idx_powerset (Int $size is copy) returns Array {
    my @c = ([]);
    for 0 .. $size-1 -> $i {
        push @c, (map { [@$_, $i] }, @c);
    }
    return @c;
}

# Fifteens
$score += 2 * grep {
    15 == [+]( @hand[@$_]>>.<val> )
}, gen_idx_powerset( +@hand );