code: func (r *sentReq) deliver(peer distPeer, msg *Msg) error {
  	r.lock.Lock()
  	defer r.lock.Unlock()
  
  	s, ok := r.sentTo[peer]
  	if !ok || s.delivered {
  		return errResp(ErrUnexpectedResponse, "reqID = %v", msg.ReqID)
  	}
  	valid := r.validate(peer, msg) == nil
  	r.sentTo[peer] = sentReqToPeer{true, s.valid}
  	s.valid <- valid
  	if !valid {
  		return errResp(ErrInvalidResponse, "reqID = %v", msg.ReqID)
  	}
  	return nil
  }
docs: // deliver a reply belonging to this request