synchronized (mutex) {
        return delegate().set(index, element);
      }
    }

    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      synchronized (mutex) {
        delegate().replaceAll(operator);
      }
    }

    @Override
    public void sort(@Nullable Comparator<? super E> c) {
      synchronized (mutex) {
        delegate().sort(c);

    try {
      unmod.putAll(Collections.singletonMap(4, "four"));
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.replaceAll((k, v) -> v);
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.putIfAbsent(3, "three");