Iterator<? extends T> a,
      Iterator<? extends T> b,
      Iterator<? extends T> c,
      Iterator<? extends T> d) {
    checkNotNull(a);
    checkNotNull(b);
    checkNotNull(c);
    checkNotNull(d);
    return concat(consumingForArray(a, b, c, d));
  }

  /**
   * Combines multiple iterators into a single iterator. The returned iterator iterates across the

      Iterator<? extends T> a,
      Iterator<? extends T> b,
      Iterator<? extends T> c,
      Iterator<? extends T> d) {
    checkNotNull(a);
    checkNotNull(b);
    checkNotNull(c);
    checkNotNull(d);
    return concat(consumingForArray(a, b, c, d));
  }

  /**
   * Combines multiple iterators into a single iterator. The returned iterator iterates across the

    }
  }

  /*
   * Preconditions.checkNotNull is *intended* for performing eager null checks on parameters that a
   * nullness checker can already "prove" are non-null. That means that the first parameter to
   * checkNotNull *should* be annotated to require it to be non-null.
   *

    }
  }

  /*
   * Preconditions.checkNotNull is *intended* for performing eager null checks on parameters that a
   * nullness checker can already "prove" are non-null. That means that the first parameter to
   * checkNotNull *should* be annotated to require it to be non-null.
   *

    @Override
    public int countIn(CharSequence sequence) {
      checkNotNull(sequence);
      return 0;
    }

    @Override
    public CharMatcher and(CharMatcher other) {
      checkNotNull(other);
      return this;
    }

    @Override
    public CharMatcher or(CharMatcher other) {
      return checkNotNull(other);
    }

    @Override
    public CharMatcher negate() {

   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @InlineMe(
      replacement = "checkNotNull(delegate)",
      staticImports = "com.google.common.base.Preconditions.checkNotNull")
  @Deprecated
  public static <K, V> Multimap<K, V> unmodifiableMultimap(ImmutableMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }

    @Override
    public int countIn(CharSequence sequence) {
      checkNotNull(sequence);
      return 0;
    }

    @Override
    public CharMatcher and(CharMatcher other) {
      checkNotNull(other);
      return this;
    }

    @Override
    public CharMatcher or(CharMatcher other) {
      return checkNotNull(other);
    }

    @Override
    public CharMatcher negate() {

   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @InlineMe(
      replacement = "checkNotNull(delegate)",
      staticImports = "com.google.common.base.Preconditions.checkNotNull")
  @Deprecated
  public static <K, V> Multimap<K, V> unmodifiableMultimap(ImmutableMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }

   * {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   */
  public static <E extends @Nullable Object> SetView<E> union(
      Set<? extends E> set1, Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    return new SetView<E>() {
      @Override
      public int size() {
        int size = set1.size();
        for (E e : set2) {
          if (!set1.contains(e)) {

   * {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   */
  public static <E extends @Nullable Object> SetView<E> union(
      Set<? extends E> set1, Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    return new SetView<E>() {
      @Override
      public int size() {
        int size = set1.size();
        for (E e : set2) {
          if (!set1.contains(e)) {