}

  public void testAsSet_present() {
    Set<String> expected = Collections.singleton("a");
    assertEquals(expected, Optional.of("a").asSet());
  }

  public void testAsSet_absent() {
    assertTrue("Returned set should be empty", Optional.absent().asSet().isEmpty());
  }

  public void testAsSet_presentIsImmutable() {
    Set<String> presentAsSet = Optional.of("a").asSet();