code: public function pipeline(PipelineInterface $pipeline = null) : PipelineInterface
      {
          if ($pipeline){
              $this->pipeline = $pipeline;
          }
          if (!$this->pipeline){
              $this->pipeline = new NullPipeline;
          }
          return $this->pipeline;
      }
docs: Set/get pipeline
  
  @param PipelineInterface $pipeline
  
  @return PipelineInterface