sub subsets {
    my $set = shift @_;
    return if length($set) == 1;
    my ($head, $char, $tail) = ($set, '', '');
    my @ret;
    while ($head) {
        $char = chop $head;
        push @ret, $head . $tail;
        $tail = $char . $tail;
    }
    return @ret;
}