Compiling Simple Context Restrictions with Nondeterministic Automata

This paper describes a non-conventional method for compiling (phonological or morpho-syntactic) context restriction (CR) constraints into non-deterministic automata in finite-state tools and surface parsing systems. The method reduces any CR into a simple one that constraints the occurrences of the empty string and represents right contexts with co-determististic states. In cases where a fully deterministic representation would be exponentially larger, this kind of inward determinism in contexts can bring benefits over various De Morgan approaches where full determinization is necessary. In the method, an accepted word gets a unique path that is a projection of a ladder-shaped structure in the context recognizer. This projection is computed in time that is polynomial to the number of context states. However, it may be difficult to take advantage of the method in a finite-state library that coerces intermediate results into canonical automata and whose intersection operation assumes deterministic automata.