Formalizing Narratives using Nested Circumscription
Chitta Baral, Alfredo Gabaldon and Alessandro Provetti
Abstract.
Representing and reasoning about narratives together with
the ability to do hypothetical reasoning is important for agents
in a dynamic world. These agents need to record their observations and
action executions as a narrative and at the same time,
to achieve their goals against a changing environment, they need to
make plans (or re-plan) from the {\em current} situation.
The early action formalisms did one or the other.
For example, while the original
situation calculus was meant for hypothetical reasoning
and planning, the event calculus was more appropriate for
narratives. Recently, there have been some attempts at developing
formalisms that do both. Independently, there has also been
a lot of recent research in reasoning about actions using
circumscription. Of particular interest to us is the research
on using high-level languages and their logical representation
using nested abnormality theories (NATs) --a form of circumscription
with blocks that make knowledge representation modular.
Starting from from theories in the high-level language $\call$, which is in passing extended to allow concurrent actions, we define a translation to
NATs that preserves both narrative and hypothetical reasoning.
We initially use the high level language $\call$, and then
extend it to allow concurrent actions. In the process, we study
several knowledge representation issues such as filtering,
and restricted monotonicity with respect to NATs.
Finally, we compare our formalization
with other approaches, and discuss how our use of NATs
makes it easier to incorporate other features of action theories,
such as constraints,
to our formalization.