code: private void addToSpanNotSet(int c) {
          if (Utility.sameObjects(spanNotSet, null) || Utility.sameObjects(spanNotSet, spanSet)) {
              if (spanSet.contains(c)) {
                  return; // Nothing to do.
              }
              spanNotSet = spanSet.cloneAsThawed();
          }
          spanNotSet.add(c);
      }
docs: Adds a starting or ending string character to the spanNotSet
  so that a character span ends before any string.