} else {
      assertThrows(UnsupportedOperationException.class, () -> keys.removeAll(singleton(key)));
    }
    assertInvariants(map);
  }

  public void testKeySetRetainAll() {
    Map<K, V> map;
    try {
      map = makePopulatedMap();
    } catch (UnsupportedOperationException e) {
      return;
    }

    Set<K> keys = map.keySet();