code: protected function validateRootsByScope($roots) {
      foreach($this->groupRootsByScope($roots) as $scope => $groupedRoots) {
        $valid = $this->isEachRootValid($groupedRoots);
  
        if ( !$valid )
          return false;
      }
  
      return true;
    }
docs: Check that each root node in the list supplied satisfies that its bounds
  values (lft, rgt indexes) are less than the next *within each scope*.
  
  @param   mixed   $roots
  @return  boolean