Syntactic-Head-Driven Generation

SYNTACTIC- I]~EAD- DRIVEN (zlENERA'I'ION Esther K6nig* Institute for Computational Linguistics, Azenbergstr.12, 70174 Stuttgart,Germany, esther@ims.uni-stuttgart.de Abstract The previously proposed semanlic-head-driw'.n Ken - eration methods run into problems if none of the daughter eonstituents in the syntact.o-semantic rule schemata of a grammar fits the definition of a semantic head given in [Shieber et al., 1990]. This is the case for the semantic analysis rnles of certain constraint- based semantic representations, e.g. Underspecified Discourse R,epresentation Structures (UI)RSs) [l!'rank and R.eyle, 1992]. Since head-driven generation in general has its me> its, we simply return to a syntactic definition of qmad' and demonstrate the feasibility of synlaclic- head-clriveu generation. In addition to its generality, a syntactic-head-driven algorithm provides a basis for a logically well-defined treatment of the nmvement of (syntactic) heads, for which only ad-hoc solutions ex- isted, so far. 1 Introduction IIead-driven generation methods combine both, top- clown search and bottom-np combination, in an ideal way. [Shieber el al., 1990] proposed to define the 'head' constituent It of phrase with category a~ on se- mantic grounds: the semantic representations of h and z are identical. This puts a strong restriction on the shape of semantic analysis rules: one of the leaves must share its semantic form with the. root node. IIowever, there are composition rules for seman- tic representations which violate this restriction, e.g. tim schemata for the construction of Underspecified Discourse It,epresentatim, Structures (UI)ILSs) [l"rank and Reyle, 1992] where, in general, the root of a tree is associated with a strictly larger semantic structure (,hal, a,ly of I, he leaves. Ill order to make at generation method available for grammars wl,ich do not follow the striet notion of a semantic head, a syntactic-head- driven generation algorithm is presented, which can be specialized to generate from UDRSs. In a second step, the method will be extended in order to han- dle the movement of (syntactic) heads in a logically well-defined manner. The (tactical) generation proble.m is tim task to generate a string from a semantic representation ac- cording to the syntax-semantics-relation defined in a given grammar. Let's assume that the latter relation *The research reported here has been funded by the Sonder- forsclmngsbereidt 340 "Sprachtheorctische GrmMlagen ffir die Computerllngulstik", a project of the German National Science Foundation I)I,'G. is stated by pairs of trees. The left tree stages a lo- cal syntactic dependeacy, i.e. the dominance relation between a root node and a set of leaf nodes and the linear precedence relation among the leaves. The right tree defines the relation among the semantic represen- tation of the root and the semantic representations of the leaves. We assume that there is a one-to-one map from the nonterminal leaf nodes of the (local) syntax tree on the le.af nodes of the. (locM) semantic deriw~tion tree. Example: np vp NP VP i i t I ,_ _ _ 5_-55, .... If one assumes a pairwise linking from M't to right then the links between the two trees can be on,itted. Although such pairs of trees are …

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