code: @Override
    public void mustReplace(final int slotNo, final int value) {
      final int idx = find(auxIntArr, lgAuxArrInts, lgConfigK, slotNo);
      if (idx >= 0) {
        auxIntArr[idx] = HllUtil.pair(slotNo, value); //replace
        return;
      }
      final String pairStr = HllUtil.pairString(HllUtil.pair(slotNo, value));
      throw new SketchesStateException("Pair not found: " + pairStr);
    }
docs: In C: two-registers.c Line 321.