Deciding Equivalence of Linear Tree-to-Word Transducers in Polynomial   Time
  We show that the equivalence of deterministic linear top-down tree-to-word transducers is decidable in polynomial time. Linear tree-to-word transducers are non-copying but not necessarily order-preserving and can be used to express XML and other document transformations. The result is based on a partial normal form that provides a basic characterization of the languages produced by linear tree-to-word transducers. 
Adrien Boiret and Raphaela Palenta
cs.FL