code: 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();
      }
    }
docs: Gets the substring of the reference string corresponding to a range of content string
  offsets.  This will include intervening characters which may not themselves correspond to
  any content string character.
  
  Will return {@link Optional#absent()} if and only if {@link #referenceString()} returns
  absent.
  
  Please refer to the class Javadoc for coverage of available substring options.