code: func (ms *modelSpec) checkModelType(model Model) error {
  	if reflect.TypeOf(model) != ms.typ {
  		return fmt.Errorf("model was the wrong type. Expected %s but got %T", ms.typ.String(), model)
  	}
  	return nil
  }
docs: // checkModelType returns an error iff model is not of the registered type that
  // corresponds to modelSpec.