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

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