@Test
void testDistinctValues() {

    final Map<String, Integer> expected = MapStream.of(entry("Einsjedan", 1)).toMap();
    final MapStream<String, Integer> ms = MapStream.of(entry("jedan", 1), entry("Eins", 1));
    final Map<String, Integer> actual = ms.distinctValues((a, b) -> a + b).toMap();
    assertEquals(expected, actual);
}