QuickChecking Patricia Trees

Patricia trees are a space-efficient, purely functional data structure, useful for efficiently implementing both integer sets and dictionaries with integer keys. In this paper we illustrate how to build a QuickCheck model of the data structure for the purpose of testing a mature OCaml library implementing it. In doing so, we encounter a subtle bug, initially inherited from a paper by Okasaki and Gill, and since then flying under the radar for almost two decades.