code: public void compress3( AutoBuffer ab ) {
      assert max() > 32;         // Expect a larger format
      assert _byteoff == 0;       // This is only set on loading a pre-existing IcedBitSet
      assert _val.length==numBytes();
      ab.put2((char)_bitoff);
      ab.put4(_nbits);
      ab.putA1(_val,_val.length);
    }
docs: Larger compression format: dump down bytes into the AutoBuffer.