Multimap<String, Integer> multimap =
        Multimaps.invertFrom(empty, ArrayListMultimap.<String, Integer>create());
    assertTrue(multimap.isEmpty());

    ImmutableMultimap<Integer, String> single =
        new ImmutableMultimap.Builder<Integer, String>().put(1, "one").put(2, "two").build();

    // copy into existing multimap
    assertSame(multimap, Multimaps.invertFrom(single, multimap));

   * @return {@code dest}
   */
  @CanIgnoreReturnValue
  public static <K extends @Nullable Object, V extends @Nullable Object, M extends Multimap<K, V>>
      M invertFrom(Multimap<? extends V, ? extends K> source, M dest) {
    checkNotNull(dest);
    for (Map.Entry<? extends V, ? extends K> entry : source.entries()) {
      dest.put(entry.getValue(), entry.getKey());
    }