code: static final HllSketchImpl promoteHeapListOrSetToHll(final CouponList src) {
      final HllArray tgtHllArr = HllArray.newHeapHll(src.lgConfigK, src.tgtHllType);
      final PairIterator srcItr = src.iterator();
      tgtHllArr.putKxQ0(1 << src.lgConfigK);
      while (srcItr.nextValid()) {
        tgtHllArr.couponUpdate(srcItr.getPair());
      }
      tgtHllArr.putHipAccum(src.getEstimate());
  
      tgtHllArr.putOutOfOrderFlag(false);
      return tgtHllArr;
    }
docs: called by CouponList.couponUpdate()