@Test
void testToConcurrentMap() {

    final Map<String, Integer> expected = refStream().collect(TO_MAP);
    final Map<String, Integer> actual = instance.toConcurrentMap(MapStream.throwingMerger());
    assertEquals(expected, actual);
    assertTrue(actual instanceof ConcurrentMap);
    assertThrows(IllegalStateException.class, () -> {
        final Object o = MapStream.of(entry("Olle", 1), entry("Olle", 2))
            .toConcurrentMap(MapStream.throwingMerger());
    });
}