{
  my $init = 'aa';
  my $start = 'a';
  my $len = length $init;

  sub next_term {
    my $ret = $init;
    my $p = $len - 1;

    while (1) {
      substr($init, $p, 1) =~ tr/acgnt/cgnta/;
      last if substr($init, $p--, 1) ne $start;
      $init = "$start$init", $len++, last if $p < 0;
    }

    return $ret;
  }
}