code: func IsSmudgeError(err error) bool {
  	if e, ok := err.(interface {
  		SmudgeError() bool
  	}); ok {
  		return e.SmudgeError()
  	}
  	if parent := parentOf(err); parent != nil {
  		return IsSmudgeError(parent)
  	}
  	return false
  }
docs: // IsSmudgeError indicates an error while smudging a files.