/**
   * Starts specifying how to combine two {@link ClosingFuture}s into a single pipeline, assuming
   * they all succeed. If any fail, the resulting pipeline will fail.
   *
   * <p>Calling this method allows you to use lambdas or method references typed with the types of
   * the input {@link ClosingFuture}s.
   *
   * @throws IllegalStateException if a {@code ClosingFuture} has already been derived from any of

   * ...
   * ImmutableMap<Color, String> colorNames =
   *     allColors.stream().collect(toImmutableMap(c -> c, c -> c.toString()));
   * }
   *
   * <p>Streams provide a more standard and flexible API and the lambdas make it clear what the keys
   * and values in the map are.
   *
   * @throws NullPointerException if any element of {@code keys} is {@code null}, or if {@code
   *     valueFunction} produces {@code null} for any key

   * ...
   * ImmutableMap<Color, String> colorNames =
   *     allColors.stream().collect(toImmutableMap(c -> c, c -> c.toString()));
   * }
   *
   * <p>Streams provide a more standard and flexible API and the lambdas make it clear what the keys
   * and values in the map are.
   *
   * @throws NullPointerException if any element of {@code keys} is {@code null}, or if {@code
   *     valueFunction} produces {@code null} for any key