{
  # ditto

  sub rx_inc_b36 {
    my $n = reverse shift;
    $n =~ s/^(Z*)([^Z])/(0 x length $1) . $to_b36[$to_num{$2}+1]/e
      or $n = (0 x length $n) . 1;
    return scalar reverse $n;
  }
}