expectContents(nonEmptyProperSubset.toRetain);
  }

  @CollectionFeature.Require(absent = SUPPORTS_REMOVE)
  @CollectionSize.Require(absent = {ZERO, ONE})
  public void testRetainAll_subsetUnsupported() {
    expectThrows(nonEmptyProperSubset);
    expectUnchanged();
  }

  // retainAll(sameElements)

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testRetainAll_sameElements() {