Visibly Pushdown Transducers with Look-Ahead

Visibly Pushdown Transducers (VPT) form a subclass of pushdown transducers. In this paper, we investigate the extension of VPT with visibly pushdown look-ahead (VPT+LA). Their transitions are guarded by visibly pushdown automata that can check whether the well-nested subword starting at the current position belongs to the language they define. First, we show that VPT+LA are not more expressive than VPT, but are exponentially more succinct. Second, we show that the class of deterministic VPT+LA corresponds exactly to the class of functional VPT, yielding a simple characterization of functional VPT. Finally, we show that while VPT+LA are exponentially more succinct than VPT, checking equivalence of functional VPT+LA is, as for VPT, Exptime-complete. As a consequence of these results, we show that for any functional VPT there is an equivalent unambiguous one