  Visibly pushdown transducers (VPTs) are visibly pushdown automata extended
with outputs. They have been introduced to model transformations of nested
words, i.e. words with a call/return structure. As trees and more generally
hedges can be linearized into (well) nested words, VPTs are a natural formalism
to express tree transformations evaluated in streaming. This paper aims at
characterizing precisely the expressive power of VPTs with respect to other
tree transducer models.
