Frequently it also turns out that although a given set of rules is not trans-
formable by the query, some other equivalent set of rules is. The following ex-
ample illustrates the point:
EXAMPLE 6
Given two rules p(a) — p(b) and p(b) p(c) and selection σ
(Α = a)v(A = c)
, we
can get the implied rule p(a) — p(c) by application of modus ponens to the
above two rules. In this case only the rule p(b) p(c) should be evaluated be-
fore the query; the implied rule could be part of the answer. Therefore, the
"closure" of this set of rules allows us to postpone one of the rules until after
the query evaluation, which was not the case for th ...
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