docs: public final Optional<UnicodeFriendlyString> referenceSubstringByContentOffsets(
        final OffsetRange<CharOffset> contentOffsets) {
      if (referenceString().isPresent()) {
        return Optional.of(referenceString().get().substringByCodePoints(
            OffsetGroupRange.from(
                startReferenceOffsetsForContentOffset(contentOffsets.startInclusive()),
                endReferenceOffsetsForContentOffset(contentOffsets.endInclusive()))
                .asCharOffsetRange()));
      } else {
        return Optional.absent();
      }
    }