code: public function di(DiContainerInterface $di = null) : DiContainerInterface
      {
          if ($di){
              $this->di = $di;
          }
          if (!$this->di){
              $this->di = new NullDi;
          }
          return $this->di;
      }
docs: Set/get di container
  
  @param DiContainerInterface $di
  
  @return DiContainerInterface