Models of Path Constraints ?

We model semistructured data as rooted edge-labeled directed graphs. A path inclusion constraint p q is satised by a semistructured data if any nodes reached by the regular query p is also reached by the regular query q. In this paper we dene the notion of an exact model: an exact model of a set of path constraints C satises the constraint p q if and only if it is implied by C. We characterize the sets C of path inclusion constraints that have an exact model. After that we give an algorithm which computes this model. We nish by studying a particular case: word equality constraints.