code: func NotFound(detail string, a ...interface{}) *ProblemDetails {
  	return &ProblemDetails{
  		Type:       MalformedProblem,
  		Detail:     fmt.Sprintf(detail, a...),
  		HTTPStatus: http.StatusNotFound,
  	}
  }
docs: // NotFound returns a ProblemDetails with a MalformedProblem and a 404 Not Found
  // status code.