Wallace is a library for dealing with nonstructural subtyping constraints. It deals with constraint solving and simplification, and it is parameterized by the definition of a type algebra. Its goal is to serve as a plug-in component in the design of a constraint-based type-checker, regardless of the programming language being analyzed.
Its theoretical foundations are described in my PhD thesis; an up-to-date formal presentation is given in this paper.
Here is the latest release (2000/02/11). Compiling requires OCaml (version 2.02 or later).