imality condition in the definition of a model of prioritized circumscription to
show that models of prioritized circumscription must satisfy the conditions (i)
and (ii) in Lemma 1 and must therefore be constructed exactly in the same way
as the models constructed in the proof of Theorem 4. This shows that every
such model must be perfect. •
COROLLARY
1
Suppose that {P
Q
,...,P
n
} and {T
Q9
...,T
m
} are two stratifications of the same
database DB. Then M is a model of CIRC(DB,P
0
> ... > P
n
) if and only if it
is a model of CIRC(DB,r
o
> ... > TJ.
Proof:
We know that the definition of a perfect model does not depend on the
choice o
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