sub traverse {
  my $start_node = HTML::TreeBuilder->new;
  $start_node->parse_file(shift);

  {
    my $counter = 'x0000';
    sub give_id {
      my $x = $_[0];
      $x->attr('id', $counter++) unless defined $x->attr('id');
      foreach my $c ($x->content_list) {
        give_id($c) if ref $c; # ignore text nodes
      }
    };
    give_id($start_node);
  }
}