Situation Calculus is arguably the most widely studied and used formalism for reasoning about action and change. The main reason for its popularity is the ability to reason about different action sequences as explicit objects. In particular, planning can be formulated as an existence problem. This paper shwos how these proeprties break down when incompelte information about the initial state and nondeterministic action effects are introduced, basically due to the fact that this incompleteness is not adeuqtely manifested on the object level. A version of Situation Calculus is presneted which adequately mdels the alternative ways the world can develop relative to a choice of actions.
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