public function single_row( $tag, $level = 0 ) {
    global $taxonomy;
    $tag = sanitize_term( $tag, $taxonomy );

    $this->level = $level;

    if ( $tag->parent ) {
      $count = count( get_ancestors( $tag->term_id, $taxonomy, 'taxonomy' ) );
      $level = 'level-' . $count;
    } else {
      $level = 'level-0';
    }

    echo '<tr id="tag-' . $tag->term_id . '" class="' . $level . '">';
    $this->single_row_columns( $tag );
    echo '</tr>';
  }