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constraints. Suppose our protagonist clears the tail pipe again after having
successfully introduced a potato. If initially starting the engine has been
assumed qualied by default, then this assumption had to be withdrawn after
having clogged the tail pipe. But the assumption of qualication is to be re-
installed in the following situation where the potato has been removed again.
For if prior to performing the two actions nothing hints at an abnormality,
then there is no reason to believe that things have been changed by these
two actions with reverse eect. This revoking qualication is granted if state
constraints of the form (3.5) are employed. To see why, suppose that initially
each
F
i
(1
i n
) and, hence,
disq
(
a
) is assumed false by default. Suppose
further that later on one particular
F
k
2fF
1
;:::;F
n
g
is initiated and that
sometime thereafter
F
k
is terminated again. Then
:disq
(
a
) must hold at
the end. This follows from persistence of each
:F
j
(1
j n
,
j 6
=
k
) and
from the fact that
V
i
=1
:F
i
implies
:disq
(
a
) according to the completed
state constraint.
Let us summarize and illustrate the discussion on conditions for abnormal
action disqualications by a fully formalized example which highlights the
essential aspects.
Example 3.4.1.
Let
D
be the plain qualication domain consisting of the
entity
pt
, the fluent names
F
=
f
runs
0
;
in
1
;
tank-empty
0
;
low-battery
0
;
engine-problem
0
;
heavy
1
; disq
(
ignite
)
0
; disq
(
insert
)
1
; disq
(
clear
)
0
g
such
that only
ignite
0
;
runs
is a non-abnormality fluent, the action names
A
=
f
insert
1
;
clear
0
g
, the action laws
transforms
f:
runs
g
into
f
runs
g
ignite
insert
(
x
)
transforms
f:
in
(
x
)
g
into
f
in
(
x
)
g
transforms
f
in
(
x
)
g
into
f:
in
(
x
)
g
clear
the three steady state constraints
[
9x:
in
(
x
)
_
tank-empty
_
low-battery
_
engine-problem
disq
(
ignite
)]
heavy
(
x
)
disq
(
insert
(
x
))
?disq
(
clear
)
and, obtainable through the influence information
I
=
f
(
(
x
)
; disq
(
(
x
)))
g
heavy
insert
[f
(
f;disq
(
)) :
f 2f
in
(
x
)
;
tank-empty
;
low-battery
;
engine-problem
gg
ignite
these ten (steady) causal relationships:
