static <K extends @Nullable Object, V extends @Nullable Object>
      TestSetGenerator<K> keySetGenerator(
          OneSizeTestContainerGenerator<Map<K, V>, Entry<K, V>> mapGenerator) {
    TestContainerGenerator<Map<K, V>, Entry<K, V>> generator = mapGenerator.getInnerGenerator();
    if (generator instanceof TestSortedMapGenerator
        && ((TestSortedMapGenerator<K, V>) generator).create().keySet() instanceof SortedSet) {