David A. Schmidt Guards, Failure, and Partiality: Dijkstra's Guarded-Command Language Formulated Topologically. 252-271 2016 Semantics, Logics, and Calculi https://doi.org/10.1007/978-3-319-27810-0_13 conf/birthday/2016nielson db/conf/birthday/nielson2016.html#Schmidt16