This module provides machinery to deal with Hindley-Milner polymorphism. It takes care of creating and instantiating schemes, and does so in an efficient way. It is parameterized with the structure S. On top of it, it adds its own structure, because it needs every unification variable to carry extra information related to generalization. Thus, it wraps the user-specified structure S in a richer structure Data, and sets up a unifier U that works with this data. It provides an abstract representation of Hindley-Milner schemes, together with several operations that allow constructing, inspecting, and instantiating schemes.