return (NavigableMap<K, Collection<V>>) super.sortedMap();
    }

    @Override
    public @Nullable Entry<K, Collection<V>> lowerEntry(@ParametricNullness K key) {
      Entry<K, Collection<V>> entry = sortedMap().lowerEntry(key);
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable K lowerKey(@ParametricNullness K key) {