if (head == null) { // empty list
      head = tail = node;
      keyToKeyList.put(key, new KeyList<K, V>(node));
      modCount++;
    } else if (nextSibling == null) { // non-empty list, add to tail
      // requireNonNull is safe because the list is non-empty.
      requireNonNull(tail).next = node;
      node.previous = tail;
      tail = node;
      KeyList<K, V> keyList = keyToKeyList.get(key);

    List<E> copy = list.subList(0, size);
    List<E> head = list.subList(0, size - 1);
    List<E> tail = list.subList(1, size);
    assertEquals(list.get(0), copy.get(0));
    assertEquals(list.get(size - 1), copy.get(size - 1));
    assertEquals(list.get(1), tail.get(0));
    assertEquals(list.get(size - 1), tail.get(size - 2));
    assertEquals(list.get(0), head.get(0));
    assertEquals(list.get(size - 2), head.get(size - 2));

    Iterable<String> tail = skip(set, 1);
    set.remove("b");
    set.addAll(newArrayList("A", "B", "C"));
    assertThat(tail).containsExactly("c", "A", "B", "C").inOrder();
  }

  public void testSkip_structurallyModifiedSkipSomeList() throws Exception {
    List<String> list = newArrayList("a", "b", "c");
    Iterable<String> tail = skip(list, 1);
    list.subList(1, 3).clear();

    Iterable<String> tail = skip(set, 1);
    set.remove("b");
    set.addAll(newArrayList("A", "B", "C"));
    assertThat(tail).containsExactly("c", "A", "B", "C").inOrder();
  }

  public void testSkip_structurallyModifiedSkipSomeList() throws Exception {
    List<String> list = newArrayList("a", "b", "c");
    Iterable<String> tail = skip(list, 1);
    list.subList(1, 3).clear();

                .named("Doubles.asList, head subList"),
            ListTestSuiteBuilder.using(new DoublesAsListTailSubListGenerator())
                .named("Doubles.asList, tail subList"),
            ListTestSuiteBuilder.using(new DoublesAsListMiddleSubListGenerator())
                .named("Doubles.asList, middle subList"));

    TestSuite suite = new TestSuite();

                .named("Shorts.asList, head subList"),
            ListTestSuiteBuilder.using(new ShortsAsListTailSubListGenerator())
                .named("Shorts.asList, tail subList"),
            ListTestSuiteBuilder.using(new ShortsAsListMiddleSubListGenerator())
                .named("Shorts.asList, middle subList"));

    TestSuite suite = new TestSuite();

                .named("Longs.asList, head subList"),
            ListTestSuiteBuilder.using(new LongsAsListTailSubListGenerator())
                .named("Longs.asList, tail subList"),
            ListTestSuiteBuilder.using(new LongsAsListMiddleSubListGenerator())
                .named("Longs.asList, middle subList"));

    TestSuite suite = new TestSuite();

                .named("Shorts.asList, head subList"),
            ListTestSuiteBuilder.using(new ShortsAsListTailSubListGenerator())
                .named("Shorts.asList, tail subList"),
            ListTestSuiteBuilder.using(new ShortsAsListMiddleSubListGenerator())
                .named("Shorts.asList, middle subList"));

    TestSuite suite = new TestSuite();

                .named("Floats.asList, head subList"),
            ListTestSuiteBuilder.using(new FloatsAsListTailSubListGenerator())
                .named("Floats.asList, tail subList"),
            ListTestSuiteBuilder.using(new FloatsAsListMiddleSubListGenerator())
                .named("Floats.asList, middle subList"));

    TestSuite suite = new TestSuite();

                .named("Floats.asList, head subList"),
            ListTestSuiteBuilder.using(new FloatsAsListTailSubListGenerator())
                .named("Floats.asList, tail subList"),
            ListTestSuiteBuilder.using(new FloatsAsListMiddleSubListGenerator())
                .named("Floats.asList, middle subList"));

    TestSuite suite = new TestSuite();