Unidirectional derivation semantics for synchronous tree-adjoining grammars

Abstract

Synchronous tree-adjoining grammars have been given two types of semantics: one based on bimorphisms and one based on synchronous derivations, in both of which the input and output trees are constructed synchronously. We introduce a third type of semantics that is based on unidirectional derivations. It derives output trees based on a given input tree and thus marks a first step towards conditional probability distributions. We prove that the unidirectional semantics coincides with the bimorphism-based semantics with the help of a strong correspondence to linear and nondeleting extended top-down tree transducers with explicit substitution. In addition, we show that stateful synchronous tree-adjoining grammars admit a normal form in which only adjunction is used. This contrasts the situation encountered in the stateless case. © 2012 Springer-Verlag.