Abstract
Linear extended top-down tree transducers (or synchronous tree-substitution grammars) are popular formal models of tree transformations that are extensively used in syntax-based statistical machine translation. The expressive power of compositions of such transducers with and without regular look-ahead is investigated. In particular, the restrictions of ź-freeness, strictness, and nondeletion are considered. The composition hierarchy turns out to be finite for all ź-free (all rules consume input) variants of these transducers except for the nondeleting ź-free transducers. The least number of transducers needed for the full expressive power of arbitrary compositions is presented. In all remaining cases (incl. the nondeleting ź-free transducers) the composition hierarchy does not collapse.
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