code: func (r *serviceRegistry) callback(method string) *callback {
  	elem := strings.SplitN(method, serviceMethodSeparator, 2)
  	if len(elem) != 2 {
  		return nil
  	}
  	r.mu.Lock()
  	defer r.mu.Unlock()
  	return r.services[elem[0]].callbacks[elem[1]]
  }
docs: // callback returns the callback corresponding to the given RPC method name.