code: @Override
    public ImmutableSet<V> get(@Nullable K key) {
      // This cast is safe as its type is known in constructor.
      ImmutableSet<V> set = (ImmutableSet<V>) map.get(key);
      return MoreObjects.firstNonNull(set, emptySet);
    }
docs: Returns an immutable set of the values for the given key. If no mappings in the multimap have
  the provided key, an empty immutable set is returned. The values are in the same order as the
  parameters used to build this multimap.