Nonmonotonic Sorts for Feature Structures

There have been many recent attempts to incorporate defaults into unification-based grammar formalisms. What these attempts have in common is that they all lose one of the most desirable properties of feature systems: namely, presentation order independence. This paper describes a method of dealing with defaults that retains order independence. The method works by making a strong distinction between strict and default information. The addition of nonmonotonic sorts allows default information to be carried in the feature structure while retaining a simple, deterministic unification operation. Monotonic feature structures are rederived through a satisfaction relation that is abstract in that it depends only on the ordering information for sorts.