public void testAsSet_presentIsImmutable() {
    Set<String> presentAsSet = Optional.of("a").asSet();
    assertThrows(UnsupportedOperationException.class, () -> presentAsSet.add("b"));
  }

  public void testAsSet_absentIsImmutable() {
    Set<Object> absentAsSet = Optional.absent().asSet();
    assertThrows(UnsupportedOperationException.class, () -> absentAsSet.add("foo"));
  }

  public void testTransform_absent() {