We conclude by noting that the function, as defined in this section, is also useful for formalizing a number of other phenomena. Examine a naive formalism for reasoning about action where the preconditions for flying are given by the formula
In common sense reasoning we want the ability to postpone dealing with the precondition of being clothed. This can be done by considering a context which assumes that one is clothed . By the importation rule and the formula 38 we get
Thus in the context we do not need to consider the precondition of being clothed in order to infer that one can fly.
Note that we are only developing an ontology for representing this phenomena, and are not dealing with pragmatic issues like which context a reasoning system will start in, and how the system will decide to consider a context making an additional assumption. In fact, from a pragmatic viewpoint the above process might need to be completely reversed. The reasoning system may realize that its current problem solving context c is making a particular assumption p that needs to be discharged. Then it will need to consider a context c' such that .
The function is also needed for representing discourse. In §8 we show how it is used to handle replies to a query; in that section we call the function `` ''.
See §7.2 for related examples in the planning domain.