code: @Override
    public SortedSet<V> get(@Nullable K key) {
      return (SortedSet<V>) super.get(key);
    }
docs: Returns a collection view of all values associated with a key. If no mappings in the multimap
  have the provided key, an empty collection is returned.
  
  <p>Changes to the returned collection will update the underlying multimap, and vice versa.
  
  <p>Because a {@code SortedSetMultimap} has unique sorted values for a given key, this method
  returns a {@link SortedSet}, instead of the {@link Collection} specified in the {@link
  Multimap} interface.