code: public function trunks() {
      $grammar = $this->getConnection()->getQueryGrammar();
  
      $rgtCol = $grammar->wrap($this->getQualifiedRightColumnName());
      $lftCol = $grammar->wrap($this->getQualifiedLeftColumnName());
  
      return $this->descendants()
                  ->whereNotNull($this->getQualifiedParentColumnName())
                  ->whereRaw($rgtCol . ' - ' . $lftCol . ' != 1');
    }
docs: Instance scope targeting all of its nested children which are between the
  root and the leaf nodes (middle branch).
  
  @return \Illuminate\Database\Query\Builder