code: private boolean isKwarg(Map<String, Object> arg) {
          Object kwarg = arg.get(KWARG_KEY);
          return kwarg != null
                  && kwarg instanceof Boolean
                  && ((Boolean) kwarg);
      }
docs: Checks whether an object argument is kwarg.
  Object argument is kwarg if it contains __kwarg__ property set to true.
  
  @param arg object argument to be tested
  @return true if object argument is kwarg