code: public AlphabeticIndex<V> addLabels(Locale... additions) {
          for (Locale addition : additions) {
              addIndexExemplars(ULocale.forLocale(addition));
          }
          buckets = null;
          return this;
      }
docs: Add more index characters (aside from what are in the locale)
  @param additions additional characters to add to the index, such as those in Swedish.
  @return this, for chaining