code: public function parentClass( $parentClass = 'undefined') {
      if( 'undefined' !== $parentClass ){
        if( ! is_object( $parentClass)){
          $this->exception('Invalid instance of a parent class.');
        } else {
          $this->traitHasParentClass = $parentClass;
        }
      }
      return $this->traitHasParentClass;
    }
docs: Parent class getter/setter
  
  @param mixed $parentClass Parent class instance
  @return mixed Parent class