Information Technology Reference
In-Depth Information
causes support the proliferation of explanations in case an abnormal disqual-
ication surprisingly occurs. Additional information may be provided as to
dierent degrees of abnormality. The latter also furnishes a ready approach
to accommodate situations in which a suitable explanation is not possible.
The default assumption of `normality' is formally captured by a model prefer-
ence criterion, which induces a nonmonotonic entailment relation|a feature
inherently connected with the Qualication Problem.
3.6 A Fluent Calculus Axiomatization
The adaptation of our Fluent Calculus-based axiomatization at the end of
Chapter 2 to qualication domains and scenarios requires two issues be ad-
dressed. First, the notion of action sequences being qualied needs renement.
The existence of a successor state is still necessary but no longer sucient
a criterion. What needs additionally be guaranteed is that no abnormal dis-
qualication is present. Modifying the axiomatization to this extent is rather
straightforward. Second, and more substantial, some qualitatively new mech-
anism needs to be introduced to account for nonmonotonicity as an intrinsic
feature of the Qualication Problem. Recall that our axiomatization of ram-
ication domains and scenarios uses pure classical logic, hence is monotonic.
The nonmonotonic framework in which it is to be embedded serves the pur-
pose of assuming away abnormality by default.
3.6.1 Axiomatizing Qualication
The concept of action sequences being qualied has been dened, in the
axiomatization of ramication in Chapter 2, by the following two axioms.
Qualied
([ ])
"
#
Result
(
a
;s
)
^
Successor
(
s; a; s
0
)
(3.6)
Qualied
([
a
ja
])
Qualied
(
a
)
^9s; s
0
(The reader may recall that
Qualied
(
a
) indicates that action sequence
a
is qualied, i.e., executable;
Result
(
a
;s
) determines state
s
as the result
of performing
a
; and
Successor
(
s; a; s
0
) is true i
s
0
is a successor state
of
s
and action
a
.) These axioms reflect the (idealistic) view that an action
can be successfully performed whenever the regular preconditions are met.
When addressing the Qualication Problem, an additional condition needs
to be accounted for: The action in question must not be abnormally disqual-
ied. This can be axiomatized with the help of the fluents
disq
(
a
). The new
denition of qualication, replacing (3.6), is as follows:
Qualied
([ ])
(3.7)
