Conclusions
work on structured text: relevant but restrictive
trees are simple: XSet = Dataguides = 1-index
(conceptually)
1-index: scales to cyclic data too
more complex queries:  2-index, T-index
T-index: space/generality tradeoff
Problem: how to use a specific T-index to answer
a given query. Query rewriting (see [ICDT'99]).
Need external-memory algorithm for
bisimulation/simulation.