@Test
void testSingletonOrElseEmpty() {

    val set = _Sets.<Integer>singletonOrElseEmpty(1);
    assertUnmodifiable(set);
    assertEquals(1, set.size());
    val emptySet = _Sets.<Integer>singletonOrElseEmpty(null);
    assertUnmodifiable(emptySet);
    assertEquals(0, emptySet.size());
}