In designing a computer system, apart from normal situations, we also need to cater for those exceptional conditions such that the computer will not go down under such special circumstances.   For betting systems, we need to cater, among others, for situations where the winning combination is unbacked for each betting pool.  Therefore, we need to establish a set of countback rules to define the next logical winning combination just in case the normal winning combination for a pool is unbacked, i.e. there is no investment backing that winning combination as a result of exceptional race results.

The following are some examples to illustrate those logically countback winning combinations when the normal winning combination is unbacked:

Win:  Pay 2nd  runner; or if unbacked the 3rd runner; or still unbacked, refund all bets.

Place:  Other placed runners share the net pool; and if all placed runners are unbacked, refund all bets.

Quinella:  Pay 1st and 3rd runners; or if unbacked 2nd and 3rd runners; or if still unbacked, refund all bets.

Quinella Place:  Other Quniella Place winning combinations share the net pool; and if all winning combinations are unbacked, refund all bets.

Tierce:  Pay 1st, 2nd and 3rd runners selected in any order; or if still unbacked, refund all bets.

Trio:  Refund all bets.

Double:  No consolation dividend will be declared.  Pay 1st in 1st leg and 2nd in 2nd leg; or if unbacked 1st in 1st leg and 3rd in 2nd leg; or if also unbacked 1st in 1st leg and ignore 2nd leg; or if still unbacked, refund all bets.

With a maxinum of 14 runners in each race in Hong Kong, the occassion for a winning combination being unbacked is very rare.  However, no matter how slim the chance of occurrence is, the betting system must be programmed to cater for all situations in accordance with the set of pre-defined countback rules.  Otherwise, the system may go down under abnormal race results.  The provision of counterback rules will prevent such a disaster.