code: public function setDefaultLeftAndRight() {
      $withHighestRight = $this->newNestedSetQuery()->reOrderBy($this->getRightColumnName(), 'desc')->take(1)->sharedLock()->first();
  
      $maxRgt = 0;
      if ( !is_null($withHighestRight) ) $maxRgt = $withHighestRight->getRight();
  
      $this->setAttribute($this->getLeftColumnName()  , $maxRgt + 1);
      $this->setAttribute($this->getRightColumnName() , $maxRgt + 2);
    }
docs: Sets default values for left and right fields.
  
  @return void