{
    my $counter = 'x0000';
    my $give_id;
    $give_id = sub {
      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);
    undef $give_id;
  }