docs: public static boolean grpContains(byte[] gcmp, int offset, int idx) {
      if(offset < 0) throw new IndexOutOfBoundsException("offset < 0: " + offset);
      if(idx < offset) return false;
      idx = idx - offset;
      int max_idx = (gcmp.length << 3) - 1;
      if(idx > max_idx) return false;
  
      return (gcmp[idx >> 3] & ((byte)1 << (idx % 8))) != 0;
    }