@ParametricNullness E value) {
    return new ConstantFunction<>(value);
  }

  private static final class ConstantFunction<E extends @Nullable Object>
      implements Function<@Nullable Object, E>, Serializable {
    @ParametricNullness private final E value;

    ConstantFunction(@ParametricNullness E value) {
      this.value = value;
    }

    @Override