* corresponding to each key appear in the same order as they are encountered.
   *
   * <p><b>{@code Stream} equivalent:</b> {@code stream.collect(Collectors.groupingBy(keyFunction))}
   * behaves similarly, but returns a mutable {@code Map<K, List<E>>} instead, and may not preserve
   * the order of entries.
   *
   * @param keyFunction the function used to produce the key for each value

   *
   * <p><b>{@code Stream} equivalent:</b> use {@code stream.collect(ImmutableMap.toImmutableMap(k ->
   * k, valueFunction))}. {@code ImmutableMap.copyOf(stream.collect(Collectors.toMap(k -> k,
   * valueFunction)))} behaves similarly, but may not preserve the order of entries.
   *
   * @throws NullPointerException if any element of this iterable is {@code null}, or if {@code
   *     valueFunction} produces {@code null} for any key