A Unification-Based Parser for Relational Grammar

We present an implemented unification-based parser for relational grammars developed within the s t ra t i f ied f ea tu re g r a m m a r (SFG) framework, which generalizes Kasper-Rounds logic to handle relational grammar analyses. We first introduce the key aspects of SFG and a lexicalized, graph-based variant of the framework suitable for implementing relational grammars. We then describe a head-driven chart parser for lexicalized SFG. The basic parsing operation is essentially ordinary feature-structure unification augmented with an operation of label unification to build the stratified features characteristic of SFG. I N T R O D U C T I O N Although the impact of relational grammar (RG) on theoretical linguistics has been substantial, it has never previously been put in a form suitable for computational use. RG's multiple syntactic strata would seem to preclude its use in the kind of monotonic, unification-based parsing system many now consider standard ([1], [11]). However, recent work by Johnson and Moss [2] on a Kasper-Rounds (KR) style logic-based formalism [5] for RG, called St ra t i f ied Fea ture G r a m m a r (S FG), has demonstrated that even RG's multiple strata are amenable to a feature-structure treatment. Based on this work, we have developed a unification-based, chart parser for a lexical version of SFG suitable for building computational relational grammars. A lexicalized SFG is simply a collection of s t ra t i f ied fea tu re graphs (Sgraphs) , each of which is anchored to a lexical item, analogous to lexicalized TAGs [10]. The basic parsing operation of the system is S-graph unif icat ion (S-unification): This is essentially ordinary feature-structure unification augmented with an operation of label unification to build the stratified features characteristic of SFG. R E L A T E D W O R K Rounds and Manaster-Ramer [9] suggested encoding multiple strata in terms of a "level" attribute, using path equations to state correspondences across strata. Unfortunately, "unchanged' relations in a stratum must be explicitly "carried over" via path equations to the next stratum. Even worse, these "carry over" equations vary from case to case. SFG avoids this problem. S T R A T I F I E D F E A T U R E G R A M M A R SFG's key innovation is the generalization of the concept ]eature to a sequence of so-called rela t ional signs (R-signs). The interpretation of a s t ra t i f ied fea ture is that each R-sign in a sequence denotes a primitive relation in different strata. 1 For instance, in Joe gave Mary tea there are, at the clause level, four sister arcs (arcs with the same source node), as shown in Figure h one arc labeled [HI with target gave, indicating gave is the head of the clause; one with label [1] and target Joe, indicating Joe is both the predicateargument, and surface subject, of the clause; one with label [3,2] and target Mary, indicating that l We use the following R-signs: 1 (subject), 2 (direct object), 3 (indirect object), 8 (chSmeur), Cat (Category), C (comp), F (flag), H (head), LOC (locative), M (marked), as well as the special Null R-signs 0 and/, explainedbelow.