Entry<K, Collection<V>> entry = sortedMap().ceilingEntry(key);
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable K ceilingKey(@ParametricNullness K key) {
      return sortedMap().ceilingKey(key);
    }

    @Override
    public @Nullable Entry<K, Collection<V>> higherEntry(@ParametricNullness K key) {