@Test
void reduce() {

    final Optional<Map.Entry<String, Integer>> expected = Optional.of(entry("jedandvatri", 1 + 2 + 3));
    final Optional<Map.Entry<String, Integer>> actual = instance.reduce((e0, e1) -> entry(e0.getKey()+e1.getKey() , e0.getValue()+e1.getValue()));
    assertEquals(expected, actual);
}