From the abstract:
This thesis presents a type system combining ML-style parametric polymorphism and subtyping, with type inference, principal types, and decidable type subsumption. Type inference is based on biunification, an analogue of unification that works with subtyping constraints.
Basically, the author has found a way to infer subtypes without using constraints, and instead via algebraic construction. Pretty interesting, tho I haven’t worked my whole way through it just yet.