sub genIterator {
    my($len, $count) = @_;
    my $cur = '';
    my $lastdig = $count - 1;
    return sub {
      if (length($cur) < $len) {
        $cur .= '0';
      } else {
        $cur =~ s/([^$lastdig])$lastdig*\z/$1 + 1/e
             or $cur = undef;
      }
      $cur;
    }
  }