code: func IsVersionConflictError(err error) (bool, error) {
  	e, ok := errs.Cause(err).(VersionConflictError)
  	if !ok {
  		return false, nil
  	}
  	return true, e
  }
docs: // IsVersionConflictError returns true if the cause of the given error can be
  // converted to an VersionConflictError, which is returned as the second result.