@Test
    void reduce2Arg() {
        assertEquals(OTHER_ELEMENT + ELEMENT, instance.reduce(OTHER_ELEMENT, (a, b) -> a + b));

        assertThrows(IllegalStateException.class, () -> instance.reduce(OTHER_ELEMENT, (a, b) -> a + b));
    }
Test of mapToInt method, of class SingletonStream.
common-parent/singletonstream/src/test/java/com/speedment/common/singletonstream/SingletonStreamTest.java