code: func IsInvalidAction(err error) bool {
  	if err == ErrInvalidAction {
  		return true
  	}
  	de, ok := err.(*DeltaError)
  	return ok && de.Err == ErrInvalidAction
  }
docs: // IsInvalidAction returns whether an error is a DeltaError
  // with the Err field equal to ErrInvalidAction.