code: func (s *stackContext) renderPublic(inner error) string {
  	switch {
  	case inner == nil:
  		return s.reason
  	case s.reason == "":
  		return inner.Error()
  	}
  	return fmt.Sprintf("%s: %s", s.reason, inner.Error())
  }
docs: // renderPublic renders the public error.Error()-style string for this frame,
  // combining this frame's Reason with the inner error.