assertThrows(UnsupportedOperationException.class, () -> units.remove(SomeEnum.B));
    assertThrows(UnsupportedOperationException.class, () -> units.add(SomeEnum.C));
  }

  public void testToImmutableEnumSet() {
    Set<SomeEnum> units = Stream.of(SomeEnum.D, SomeEnum.B).collect(toImmutableEnumSet());

    assertThat(units).containsExactly(SomeEnum.B, SomeEnum.D).inOrder();
  }