Comparison of XTAG and LEXSYS grammars

We use the Lexicalised D-Tree Grammar (LDTG) formalism (Rambow et al. 95), which is based on the Lexicalized Tree Adjoining Grammar (LTAG) formalism. In LDTG, there are two types of edges between nodes: d-edges, represented with a broken line, and p-edges, represented by a solid line. Trees are combined by two substitution-like operations, both of which involve combining two descriptions, by equating exactly one node from each description. One of the operations is always used to add complements, and involves equating a frontier node (in the d-tree that is getting the complement) with the root of some component (in the d-tree that is providing the complement ), such that the two nodes being equated are compatible . An example of substitution is shown in Figure 1.