Relatively Complete Verification of Probabilistic Programs

We study a syntax for specifying quantitative "assertions" - functions mapping program states to numbers - for probabilistic program verification. We prove that our syntax is expressive in the following sense: Given any probabilistic program $C$, if a function $f$ is expressible in our syntax, then the function mapping each initial state $σ$ to the expected value of $f$ evaluated in the final states reached after termination of $C$ on $σ$ (also called the weakest preexpectation $\textit{wp} [C](f)$) is also expressible in our syntax. As a consequence, we obtain a relatively complete verification system for reasoning about expected values and probabilities in the sense of Cook: Apart from proving a single inequality between two functions given by syntactic expressions in our language, given $f$, $g$, and $C$, we can check whether $g \preceq \textit{wp} [C] (f)$.