Note that over time, the repair cost for an open condition that is not resolved may either increase, as new steps that might achieve the condition are added to the plan, or decrease, as steps already in the plan are constrained by temporal ordering or variable binding so that they can no longer achieve the condition. In considering the cost of threat repair, it is useful to distinguish between nonseparable and separable threats. Of course, already existing temporal ordering constraints may block one or both or these repair options, which is why there are at most two possible repairs. Over time, the repair cost for an unresolved nonseparable threat can only decrease. In such circumstances, the threat may disappear if a subsequent variable binding blocks the unification.
Separation can introduce one repair for each unbound variable in the threat.
As with nonseparable threats, the repair cost for a separable threat that remains unresolved can only decrease over time.