code: protected function isEachRootValid($roots) {
      $left = $right = 0;
  
      foreach($roots as $root) {
        $rootLeft   = $root->getLeft();
        $rootRight  = $root->getRight();
  
        if ( !($rootLeft > $left && $rootRight > $right) )
          return false;
  
        $left   = $rootLeft;
        $right  = $rootRight;
      }
  
      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.
  
  @param   mixed   $roots
  @return  boolean