my ($do_search1, $do_search2);
    $do_search2 =  sub {

      my ($node) = @_;
      $sub->($node);
      $explored->{$node->{_id}}++;
      foreach my $link (@{$node->{_outlinks}}) {
          die "too deep" unless ($explored->{$link->{_id}});
      }
    };
    $do_search1 =  sub {

      my ($node) = @_;
      $sub->($node);
      $explored->{$node->{_id}}++;
      foreach my $link (@{$node->{_outlinks}}) {
        $do_search2->($link->{_to}) unless ($explored->{$link->{_id}});
      }
    };
    $do_search =  sub {

      my ($node) = @_;
      $sub->($node);
      $explored->{$node->{_id}}++;
      foreach my $link (@{$node->{_outlinks}}) {
        $do_search1->($link->{_to}) unless ($explored->{$link->{_id}});
      }
    };