code: func IsErrNotImplemented(err error) bool {
  	if _, ok := err.(notImplementedError); ok {
  		return ok
  	}
  	return errdefs.IsNotImplemented(err)
  }
docs: // IsErrNotImplemented returns true if the error is a NotImplemented error.
  // This is returned by the API when a requested feature has not been
  // implemented.