sub combinations {
  my (@comb, $pos);
  my $limit = 2**@_;

  while (my $item = shift) {
    my $step = 2**@_;
    for ($pos = 0; $pos < $limit; $pos += $step) {
      push @{$comb[$pos++]}, $item for 1..$step;
    }
  }

  return @comb, [];
}