code: public static SystemInputDef getResponseInputModel( OpenAPI api, ModelOptions options)
      {
      ResponseInputModeller inputModeller = new ResponseInputModeller( options);
      return inputModeller.getResponseInputModel( api);
      }
docs: Returns a {@link SystemInputDef system input definition} for the API responses defined by the given
  OpenAPI specification. Returns null if the given spec defines no API responses to model.