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