code: FormattedListBuilder format(Collection<?> items, int index) {
          Iterator<?> it = items.iterator();
          int count = items.size();
          switch (count) {
          case 0:
              return new FormattedListBuilder("", false);
          case 1:
              return new FormattedListBuilder(it.next(), index == 0);
          case 2:
              return new FormattedListBuilder(it.next(), index == 0).append(two, it.next(), index == 1);
          }
          FormattedListBuilder builder = new FormattedListBuilder(it.next(), index == 0);
          builder.append(start, it.next(), index == 1);
          for (int idx = 2; idx < count - 1; ++idx) {
              builder.append(middle, it.next(), index == idx);
          }
          return builder.append(end, it.next(), index == count - 1);
      }
docs: the offset.