Models Supporting Nondeterminism and Probabilistic Choice

In this paper we describe the problems encountered in building a semantic model that supports both nondeterministic choice and probabilistic choice. Several models exist that support both of these constructs, but none that we know of satis es all the laws one would like. Using domain-theoretic techniques, we show how a model can be devised \on top of" certain models for probabilistic choice, so that the expected laws for nondeterministic choice and probabilistic choice remain valid.