docs: def serialize_problem(req, resp, problem):
      """Serialize the given instance of Problem."""
      preferred = req.client_prefers(
          ('application/json', 'application/problem+json')
      )
      if preferred is None:
          preferred = 'application/json'
  
      resp.data = problem.to_json().encode('utf-8')
      resp.content_type = preferred
      resp.append_header('Vary', 'Accept')