code: void addSourceTargetSet(UnicodeSet filter, UnicodeSet sourceSet, UnicodeSet targetSet) {
          UnicodeSet currentFilter = new UnicodeSet(filter);
          UnicodeSet revisiting = new UnicodeSet();
          int count = ruleVector.size();
          for (int i=0; i<count; ++i) {
              TransliterationRule r = ruleVector.get(i);
              r.addSourceTargetSet(currentFilter, sourceSet, targetSet, revisiting.clear());
              currentFilter.addAll(revisiting);
          }
      }
docs: TODO Merge into r.addSourceTargetSet, to avoid duplicate testing