import org.checkerframework.checker.nullness.qual.Nullable;

@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public class Helpers {
  // Clone of Objects.equal
  static boolean equal(@Nullable Object a, @Nullable Object b) {
    return a == b || (a != null && a.equals(b));
  }

  // Clone of Lists.newArrayList
  public static <E extends @Nullable Object> List<E> copyToList(Iterable<? extends E> elements) {

  @Generates
  static <K, V> Map<K, V> generateMap(@Nullable K key, @Nullable V value) {
    return generateHashdMap(key, value);
  }

  @Generates
  static <K, V> HashMap<K, V> generateHashdMap(@Nullable K key, @Nullable V value) {
    return generateLinkedHashMap(key, value);
  }

  @Generates
  static <K, V> LinkedHashMap<K, V> generateLinkedHashMap(@Nullable K key, @Nullable V value) {

            MultimapBuilder.linkedHashKeys().linkedHashSetValues()::<K, V>build),
        ImmutableSetMultimap::copyOf);
  }

  static <
          T extends @Nullable Object,
          K extends @Nullable Object,
          V extends @Nullable Object,
          M extends Multimap<K, V>>
      Collector<T, ?, M> toMultimap(
          Function<? super T, ? extends K> keyFunction,

    }

    @Override
    Map<R, @Nullable V> getValue(int index) {
      return new Column(index);
    }

    @Override
    Map<R, @Nullable V> setValue(int index, Map<R, @Nullable V> newValue) {
      throw new UnsupportedOperationException();
    }

    @Override
    @CheckForNull
    public Map<R, @Nullable V> put(C key, Map<R, @Nullable V> value) {
      throw new UnsupportedOperationException();

   */
  public static <E extends @Nullable Object> List<E> asList(
      @ParametricNullness E first, @ParametricNullness E second, E[] rest) {
    return new TwoPlusArrayList<>(first, second, rest);
  }

  /** @see Lists#asList(Object, Object[]) */
  private static class OnePlusArrayList<E extends @Nullable Object> extends AbstractList<E>
      implements Serializable, RandomAccess {

   */
  public static <A extends @Nullable Object, B extends @Nullable Object, C extends @Nullable Object>
      Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f) {
    return new FunctionComposition<>(g, f);
  }

  private static class FunctionComposition<
          A extends @Nullable Object, B extends @Nullable Object, C extends @Nullable Object>
      implements Function<A, C>, Serializable {

                return (from == null) ? true : null;
              }
            });

    Set<Entry<@Nullable String, @Nullable Boolean>> entries = map.entrySet();
    assertTrue(entries.contains(Maps.immutableEntry("a", true)));
    assertTrue(entries.contains(Maps.<String, @Nullable Boolean>immutableEntry("b", null)));
    assertTrue(

    public static Object good(
        String a,
        int b,
        // oneConstantOnly doesn't matter since it's not nullable and can be only 1 value.
        @SuppressWarnings("unused") OneConstantEnum oneConstantOnly,
        // noConstant doesn't matter since it can only be null
        @SuppressWarnings("unused") @Nullable NoConstantEnum noConstant) {
      return new GoodEquals(a, b);
    }

    // instance method ignored

    List<@Nullable Integer> list4 = Lists.newArrayList(1, 2);
    List<@Nullable Integer> list5 = Lists.newArrayList(1, null, 2);
    List<@Nullable Integer> list6 = Lists.newArrayList(2);
    List<@Nullable Integer> list7 = Lists.newArrayList(nullInt);
    List<@Nullable Integer> list8 = Lists.newArrayList(nullInt, nullInt);
    List<@Nullable List<@Nullable Integer>> list =

    List<@Nullable Integer> list4 = Lists.newArrayList(1, 2);
    List<@Nullable Integer> list5 = Lists.newArrayList(1, null, 2);
    List<@Nullable Integer> list6 = Lists.newArrayList(2);
    List<@Nullable Integer> list7 = Lists.newArrayList(nullInt);
    List<@Nullable Integer> list8 = Lists.newArrayList(nullInt, nullInt);
    List<@Nullable List<@Nullable Integer>> list =