code: @Override
      public void characters(char[] ch, int start, int length)
          throws SAXException {
          if (vIsOpen) {
              value.append(ch, start, length);
          }
          if (fIsOpen) {
              formula.append(ch, start, length);
          }
          if (hfIsOpen) {
              headerFooter.append(ch, start, length);
          }
      }
docs: Captures characters only if a suitable element is open.
  Originally was just "v"; extended for inlineStr also.