code: public int unmapClass(int clazz) {
      Col c = _c[_c.length-1];
      if (c._isByte)
        return clazz;
      else {
        // OK, this is not fully correct bad handle corner-cases like for example dataset uses classes only
        // with 0 and 3. Our API reports that there are 4 classes but in fact there are only 2 classes.
        if (clazz >= c._binned2raw.length) clazz = c._binned2raw.length - 1;
        return (int) (c.raw(clazz) - c._min);
      }
    }
docs: Transforms given binned index (short) from class column into a value from interval [0..N-1]
  corresponding to a particular predictor class