* there's no way to assign a {@code @Nullable T} to a field {@code T foo;} and instruct the
   * analysis that that means "plain {@code T}" rather than the inferred type {@code @Nullable T}.
   * (Even if supported added {@code @NonNull}, that would not help, since the problem case
   * addressed by this method is the case in which {@code T} has parametric nullness -- and thus its
   * value may be legitimately {@code null}.)
   */

   * there's no way to assign a {@code @Nullable T} to a field {@code T foo;} and instruct the
   * analysis that that means "plain {@code T}" rather than the inferred type {@code @Nullable T}.
   * (Even if supported added {@code @NonNull}, that would not help, since the problem case
   * addressed by this method is the case in which {@code T} has parametric nullness -- and thus its
   * value may be legitimately {@code null}.)
   */
  @ParametricNullness

   * there's no way to assign a {@code @Nullable T} to a field {@code T foo;} and instruct the
   * analysis that that means "plain {@code T}" rather than the inferred type {@code @Nullable T}.
   * (Even if supported added {@code @NonNull}, that would not help, since the problem case
   * addressed by this method is the case in which {@code T} has parametric nullness -- and thus its
   * value may be legitimately {@code null}.)
   */
  @ParametricNullness

      @SuppressWarnings({"nullness", "unchecked"}) // see supertype
      public ImmutableSet<@NonNull E> immutableCopy() {
        ImmutableSet.Builder<@NonNull E> builder =
            new ImmutableSet.Builder<@NonNull E>()
                .addAll((Iterable<@NonNull E>) set1)
                .addAll((Iterable<@NonNull E>) set2);
        return (ImmutableSet<@NonNull E>) builder.build();
      }
    };
  }

  /**

    // TODO(lowasser): consider an implementation that can support SUBSIZED
    boolean isParallel = false;
    int characteristics = Spliterator.ORDERED | Spliterator.SIZED | Spliterator.NONNULL;
    long estimatedSize = 0L;
    ImmutableList.Builder<Spliterator<? extends T>> splitrsBuilder =
        new ImmutableList.Builder<>(streams.length);
    for (Stream<? extends T> stream : streams) {

    public static MavenOptions layerMavenOptions(Collection<MavenOptions> options) {
        List<MavenOptions> o = options.stream().filter(Objects::nonNull).toList();
        if (o.isEmpty()) {
            throw new IllegalArgumentException("No options specified (or all were null)");
        } else if (o.size() == 1) {
            return o.get(0);
        } else {

        strategy.forEach(spliteratorSupplier.get(), resultsForStrategy::add);

        // TODO(cpovirk): better failure messages
        if ((characteristics & Spliterator.NONNULL) != 0) {
          assertFalse(resultsForStrategy.contains(null));
        }
        if ((characteristics & Spliterator.SORTED) != 0) {
          Comparator<? super E> comparator = spliterator.getComparator();

        strategy.forEach(spliteratorSupplier.get(), resultsForStrategy::add);

        // TODO(cpovirk): better failure messages
        if ((characteristics & Spliterator.NONNULL) != 0) {
          assertFalse(resultsForStrategy.contains(null));
        }
        if ((characteristics & Spliterator.SORTED) != 0) {
          Comparator<? super E> comparator = spliterator.getComparator();

        return delegate().compute(key, remappingFunction);
      }
    }

    @Override
    @CheckForNull
    public V merge(
        K key,
        @NonNull V value,
        BiFunction<? super @NonNull V, ? super @NonNull V, ? extends @Nullable V>
            remappingFunction) {
      synchronized (mutex) {
        return delegate().merge(key, value, remappingFunction);
      }
    }

        new ListFuture<V>(ImmutableList.copyOf(futures), true);
    // allAsList ensures that it fills the output list with V instances.
    @SuppressWarnings("nullness")
    ListenableFuture<List<V>> nonNull = nullable;
    return nonNull;
  }

  /**
   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
   * input futures, if all succeed.
   *