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C00002 00002	now[w86,jmc]		Use of indexicals in controlling inference
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now[w86,jmc]		Use of indexicals in controlling inference

Suppose we introduce a predicate  now(s)  which indicates that
a certain situation is the current situation.  We suppose that
certain sentences giving the effects of actions have  now(s)  as
a condition.  Then only that situation will be reasoned about
in these rules.  This applies whether we talk about OPS5-type rules
or Prolog sentences.  Actually we can have several  now-like
predicates, and let some of them apply to more than one situation
or other entity.  What situation we regard as  now  can depend on
meta-level reasoning concerning goals.  More generally we have
interest predicates whose applicability to any kind of object is
a premiss of certain rules and is altered by program or meta-reasoning.

Another way of doing this in Prolog is to add a premiss to the
right hand side of each ordinary rule of the form

ok-to-use(r7,SS)

which means that it is ok to use this rule, call it r7, in situation  
SS.  We might even write it

ok-to-use(r7,NOW).

The proposition  ok-to-use(r7,NOW)  has to be established by other
reasoning.  The reasoning is conceptually meta-level, although it
is actually carried out on the same level as the other reasoning.