Probabilistic programming languages are used to write probabilistic models to make probabilistic inferences. A number of rigorous semantics have recently been proposed that are now available to carry out formal verification of probabilistic programs. In this paper, we extend an existing formalization of measure and integration theory with s-finite kernels, a mathematical structure to interpret typing judgments in the semantics of a probabilistic programming language. The resulting library makes it possible to reason formally about transformations of probabilistic programs and their execution.