code: public static final Union heapify(final Memory mem) {
      final int lgK = HllUtil.checkLgK(mem.getByte(PreambleUtil.LG_K_BYTE));
      final HllSketch sk = HllSketch.heapify(mem);
      final Union union = new Union(lgK);
      union.update(sk);
      return union;
    }
docs: Construct a union operator populated with the given Memory image of an HllSketch.
  @param mem the given Memory
  @return a union operator populated with the given Memory image of an HllSketch.