# Add bookmarks.
    my $outline = $ol_root->outline;
    $outline->title("Bookmarks");
    $outline->closed;

    my @tops =
      map  { $_->[0] }
      sort { $a->[1] cmp $b->[1] }
      map  { [ $_ => s/^song_([0-9]+)$/sprintf("song_%06d",$1)/er ] }
      grep { ! /^(?:cover|front|toc|back)$/ }
      keys %{ $self->{_nd} };

    for ( "cover", "front", "toc", @tops, "back" ) {
	next unless my $p = $self->{_nd}->{$_};
	my $ol = $outline->outline;
	$ol->title($_);
	if ( my $c = $ol->can("destination") ) {
	    $c->( $ol, $p );
	}
	else {
	    $ol->dest($p);
	}
    }