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.
