*
   * <p>This interface is only intended for use by callers of {@link #mapWithIndex(Stream,
   * FunctionWithIndex)}.
   *
   * @since 33.4.0 (but since 21.0 in the JRE flavor)
   */
  public interface FunctionWithIndex<T extends @Nullable Object, R extends @Nullable Object> {
    /** Applies this function to the given argument and its index within a stream. */
    @ParametricNullness