code: private final static boolean checkExpand(TypedArrayDecoder tad)
      {
          if (tad instanceof ValueDecoderFactory.BaseArrayDecoder) {
              ((ValueDecoderFactory.BaseArrayDecoder) tad).expand();
              return true;
          }
          return false;
      }
docs: Internal method used to see if we can expand the buffer that
  the array decoder has. Bit messy, but simpler than having
  separately typed instances; and called rarely so that performance
  downside of instanceof is irrelevant.