Local tree description grammars. A local extension of TAG allowing underspecified dominance relations (Q5949343)

In this paper, a new grammar formalism called local Tree Description Grammar (TDG) has been proposed. Local TDGs are a formalism generating tree descriptions. The grammar consists of elementary descriptions, and from these elementary descriptions, larger descriptions are generated with a local derivation mechanism. Local TDGs preserve some of the attractive properties of Tree Adjoining Grammars (TAG) with respect to natural languages, in particular the extended domain of locality and the possibility of lexicalization. Furthermore, they allow for multicomponent derivation and the formalization of dominance constraints. Both properties are necessary for dealing with natural languages. Concerning the generative capacity of local TDGs, it is shown that local TDGs generate only semilinear languages. This indicates that local TDGs are suitable for describing natural languages, since the languages generated by local TDGs satisfy the constant growth property. Furthermore, local TDGs are at least as powerful as et-local MC-TAGs. Their derivational generative capacity is even greater than that of set-local MC-TAGs. A subject that has been left aside in this paper is the problem of parsing and in particular parsing complexity. Examples of scope ambiguities and scrambling have shown that local TDGs allow the generation of tree descriptions with an underspecified dominance relation. This is useful for several reasons. On the one hand, the fact that the construction of minimal trees is not restricted by any locality condition, combined with the possibility of underspecified dominance relation, provides a sufficient degree of non-locality for analyzing scrambling phenomena, while at the same time generating a derivation structure that represents an adequate dependency structure. This was not possible with previous proposals for scrambling within non-local MC-TAG or V-TAG. Summarizing, one can say that local TDGs combine some of the advantages of other TAG variants with a local derivation structure and the possibility of relaxing not only the parent but also the dominance relations. The examples have shown that these properties are very useful in dealing with natural languages.