public FEELFnResult<String> invoke(@ParameterName("string") String string, @ParameterName("start position") Number start) {
        return invoke(string, start, null);
    }
@Test
    void invokeStartPositive() {
        FunctionTestUtil.assertResult(substringFunction.invoke("test", 1), "test");
        FunctionTestUtil.assertResult(substringFunction.invoke("test", 2), "est");
        FunctionTestUtil.assertResult(substringFunction.invoke("test", 4), "t");

        FunctionTestUtil.assertResult(substringFunction.invoke("test", 2, 1), "e");
        FunctionTestUtil.assertResult(substringFunction.invoke("test", 2, 2), "es");
    }