return sortedMap().ceilingKey(key);
    }

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

    @Override
    @CheckForNull
    public K higherKey(@ParametricNullness K key) {

      return sortedMap().ceilingKey(key);
    }

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

    @Override
    @CheckForNull
    public K higherKey(@ParametricNullness K key) {