  Some of the most relevant document schemas used online, such as XML and JSON,
have a nested format. In the last decade, the task of extracting data from
nested documents over streams has become especially relevant. We focus on the
streaming evaluation of queries with outputs of varied sizes over nested
documents. We model queries of this kind as Visibly Pushdown Transducers (VPT),
a computational model that extends visibly pushdown automata with outputs and
has the same expressive power as MSO over nested documents. Since processing a
document through a VPT can generate a massive number of results, we are
interested in reading the input in a streaming fashion and enumerating the
outputs one after another as efficiently as possible, namely, with
constant-delay. This paper presents an algorithm that enumerates these elements
with constant-delay after processing the document stream in a single pass.
Furthermore, we show that this algorithm is worst-case optimal in terms of
update-time per symbol and memory usage.
