Any plan produced from the lifting axiom 70 makes numerous assumptions. For example, it assumes that the shortest path will always get the cargo to its destination on time. Although this assumption is usually valid, we can imagine a scenario in which an urgent delivery will need to take a longer route in order to get to its destination on time. We thus need to consider the timeliness of a path in scenarios which involve urgent deliveries.
In robotics, assumptions of this sort are commonly called kindness assumptions, cf. , because they amount to assuming that the world is kind, i.e. that things will turn out in our favor most of the time. Kindness assumptions are a useful tool and are commonly made both when constructing and integrating plans. They allow us to focus on the aspects of a plan that seem to be relevant to the problem at hand and to disregard details which we assume will hold for that particular problem class. However, whenever kindness assumptions are made it is important to have a mechanism which enables us to discharge such assumptions and reason about their validity in cases when it is unclear whether they hold. The context formalisms enables us to do this in the framework of logic.
Assume that after deriving the plan in formula 69 (by integrating the plans from the route and supply planning contexts) we realize that the delivery is needed urgently. At this point our goal is to discharge the timeliness assumption and take the proposed path through NYC only if it gets equipment1 to Frankfurt on time. The desired plan, which is given in an urgent problem solving context , is thus
where deciding whether holds will involve looking up airplane schedules and local delivery facilities in some data base. We are assuming that conditional plans, like formula 71, can be represented by the system. In the general case, formula 71 follows from formula 69 and the lifting axiom
In some planning instances we will want to consider the timeliness issues at the very outset. We can avoid using the original problem solving context by inferring a lifting theorem which integrates a plan from the route planner and a plan from the supply planner to directly produce a plan in
Formula 73 logically follows from the lifting axioms given in formula 70 and formula 72.