* stream.collect(ImmutableMap.toImmutableMap(keyFunction, v -> v))}.
   *
   * @param keyFunction the function used to produce the key for each value
   * @return a map mapping the result of evaluating the function {@code keyFunction} on each value
   *     in this fluent iterable to that value
   * @throws IllegalArgumentException if {@code keyFunction} produces the same key for more than one

   * this may not preserve the order of entries.
   *
   * @param keyFunction the function used to produce the key for each value
   * @return a map mapping the result of evaluating the function {@code keyFunction} on each value
   *     in this fluent iterable to that value
   * @throws IllegalArgumentException if {@code keyFunction} produces the same key for more than one