It is generally accepted that a distinction may be drawn between belief revision and belief updating. As the authors of the paper under review put it, ``loosely speaking, the former says that the beliefs may have been wrong and in need of revision, whereas the latter says that the beliefs were correct, but the world has in the meanwhile evolved and the beliefs must be updated''. \textit{G. Grahne}, \textit{A. Mendelzon} and \textit{R. Reiter} [``On the semantics of belief revision systems'', Proc. Third Conference on Theoretical Aspects of Reasoning and Knowledge (1992)] suggested a possible reduction of the concept of revision to that of update, by treating a revision of a belief set as an update of the mental state that carries it. The authors of the paper under review presents another reduction in the same spirit, but in which update itself is expressed in the language of ``situation calculus'' which makes explicit reference to actions, and time, unlike the Katsuno-Mendelzon language for update, which is purely propositional.