private function build_section_sub_tree($main_tree, $tree_level, $section_level) {
    $sub_sections = array();
    if (isset($main_tree[$tree_level]) && isset($main_tree[$tree_level][$section_level])) {
      $sub_sections = $main_tree[$tree_level][$section_level];
      $sub_tree_level = $tree_level + 1;
      foreach ($sub_sections as $sub_section_level => $sub_section) {
        $sub_sections[$sub_section_level]['sub'] = $this->build_section_sub_tree($main_tree, $sub_tree_level, $sub_section_level);
      }
    }
    return $sub_sections;
  }