code: func (f *Fetcher) Enqueue(peer string, block *types.Block) error {
  	op := &inject{
  		origin: peer,
  		block:  block,
  	}
  	select {
  	case f.inject <- op:
  		return nil
  	case <-f.quit:
  		return errTerminated
  	}
  }
docs: // Enqueue tries to fill gaps the fetcher's future import queue.