code: @Override
    public void mustAdd(final int slotNo, final int value) {
      final int index = find(auxIntArr, lgAuxArrInts, lgConfigK, slotNo);
      final int pair = HllUtil.pair(slotNo, value);
      if (index >= 0) {
        final String pairStr = HllUtil.pairString(pair);
        throw new SketchesStateException("Found a slotNo that should not be there: " + pairStr);
      }
        //Found empty entry
        auxIntArr[~index] = pair;
        auxCount++;
        checkGrow();
    }
docs: In C: two-registers.c Line 300.