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