%T A Predicate Transformer Semantics for a Concurrent Language of Refinement
%A  Ana Cavalcanti,  Jim Woodcock
%E  James S. Pascoe,  Roger J. Loader,  Vaidy S. Sunderam
%B Communicating Process Architectures 2002
%X Circus is a combination of Z and CSP; its chief
   distinguishing feature is the inclusion of the ideas of the
   refinement calculus. Our main objective is the definition of
   refinement methods for concurrent programs. The original
   semantic model for Circus is Hoare and He\[rs]s unifying
   theories of programming. In this paper, we present an
   equivalent semantics based on predicate transformers. With
   this new model, we provide a more adequate basis for the
   formalisation of refinement and verification\-condition
   generation rules. Furthermore, this new framework makes it
   possible to include logical variables and angelic
   nondeterminism in Circus. The consistency of the relational
   and predicate transformer models gives us confidence in
   their accuracy.