code: @Override
    @GwtIncompatible // NavigableMap
    NavigableMap<K, Collection<V>> backingMap() {
      return (NavigableMap<K, Collection<V>>) super.backingMap();
    }
docs: /*
  The following @GwtIncompatible methods override the methods in
  AbstractSortedKeySortedSetMultimap, so GWT will fall back to the ASKSSM implementations,
  which return SortedSets and SortedMaps.