 Rather than restate Mackey’s axioms verbatim, I shall paraphrase them in the context of an approach to generalized probability theory due to D. J. Foulis and C. H. Randall having—among the many more or less analogous approaches  available[8]—certain  advantages of simplicity and flexibility. References for this section are Foulis, Greechie, and Rüttimann [1992]; Foulis, Piron and Randall [1983]; Randall and Foulis [1983]; see also Gudder [1989]; Wilce [2000b] and Wilce [2009] for surveys.