code: public static <K0> MultimapBuilderWithKeys<K0> treeKeys(final Comparator<K0> comparator) {
      checkNotNull(comparator);
      return new MultimapBuilderWithKeys<K0>() {
        @Override
        <K extends K0, V> Map<K, Collection<V>> createMap() {
          return new TreeMap<>(comparator);
        }
      };
    }
docs: Uses a {@link TreeMap} sorted by the specified comparator to map keys to value collections.
  
  <p>The collections returned by {@link Multimap#keySet()}, {@link Multimap#keys()}, and {@link
  Multimap#asMap()} will iterate through the keys in sorted order.
  
  <p>For all multimaps generated by the resulting builder, the {@link Multimap#keySet()} can be
  safely cast to a {@link java.util.SortedSet}, and the {@link Multimap#asMap()} can safely be
  cast to a {@link java.util.SortedMap}.
  
  <p>Multimaps generated by the resulting builder will not be serializable if {@code comparator}
  is not serializable.