sub gen_seqs
{
  my ($n) = @_;

  my @seqs = ();

  if($n == 1) {
    $seqs[0] = [0];
    $seqs[1] = [1];
  } else {
    my @seqs_0 = &gen_seqs($n-1);
    for my $seq (@seqs_0) {
      unshift @$seq, 0;
    }
    my @seqs_1 = &gen_seqs($n-1);
    for my $seq (@seqs_1) {
      unshift @$seq, 1;
    }
    push @seqs, @seqs_0, @seqs_1;
  }
  return @seqs;
}