@Test
public 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");
}