code: List<FilterUtils> combineFilterUtils(final Element topicref, final List<FilterUtils> filters,
                                           final SubjectScheme subjectSchemeMap) {
          return getChildElement(topicref, DITAVAREF_D_DITAVALREF)
                  .map(ditavalRef -> getFilterUtils(ditavalRef).refine(subjectSchemeMap))
                  .map(f -> {
                      final List<FilterUtils> fs = new ArrayList<>(filters.size() + 1);
                      fs.addAll(filters);
                      fs.add(f);
                      return fs;
                  })
                  .orElse(filters);
      }
docs: Combine referenced DITAVAL to existing list and refine with subject scheme.