docs: protected Map<String, Object> fmSessionModel(Optional<Session> session) {
          @SuppressWarnings("PMD.UseConcurrentHashMap")
          final Map<String, Object> model = new HashMap<>();
          Locale locale = session.map(
              sess -> sess.locale()).orElse(Locale.getDefault());
          model.put("locale", locale);
          final ResourceBundle resourceBundle = resourceBundle(locale);
          model.put("resourceBundle", resourceBundle);
          model.put("_", new TemplateMethodModelEx() {
              @Override
              @SuppressWarnings("PMD.EmptyCatchBlock")
              public Object exec(@SuppressWarnings("rawtypes") List arguments)
                      throws TemplateModelException {
                  @SuppressWarnings("unchecked")
                  List<TemplateModel> args = (List<TemplateModel>) arguments;
                  if (!(args.get(0) instanceof SimpleScalar)) {
                      throw new TemplateModelException("Not a string.");
                  }
                  String key = ((SimpleScalar) args.get(0)).getAsString();
                  try {
                      return resourceBundle.getString(key);
                  } catch (MissingResourceException e) {
                      // no luck
                  }
                  return key;
              }
          });
          return model;
      }