code: public void set(int idx) {
      idx -= _bitoff;
      assert (idx >= 0 && idx < _nbits): "Must have "+_bitoff+" <= idx <= " + (_bitoff+_nbits-1) + ": " + idx;
      _val[_byteoff+(idx >> 3)] |= ((byte)1 << (idx & 7));
    }
docs: Activate the bit specified by the integer (must be from 0 ... _nbits-1)
  @param idx -