code: func (ctl *Controller) getFactory(id ID) (Factory, error) {
  	ctl.m.RLock()
  	defer ctl.m.RUnlock()
  	if m, ok := ctl.mappers[id]; ok {
  		return m, nil
  	}
  	return nil, errors.Reason("no mapper factory with ID %q registered", id).Err()
  }
docs: // getFactory returns a registered mapper factory or an error.
  //
  // Grabs the reader lock inside. Can return only fatal errors.