@SuppressWarnings("unused") // called by NullPointerTester
    public void checkArray(FromTo<String, FromTo<Integer, String>> f, String s) {
      calledWith(f, s);
    }

    void check() {
      runTester();
      FromTo<?, ?> defaultFunction = (FromTo<?, ?>) getDefaultParameterValue(0);
      FromTo<?, ?> returnValue = (FromTo<?, ?>) defaultFunction.apply(null);
      assertEquals("", returnValue.apply(null));
    }