Francesco Bruera Serafino Cicerone Gianlorenzo D'Angelo Gabriele Di Stefano Daniele Frigioni Maintenance of Multi-level Overlay Graphs for Timetable Queries. 2007 conf/atmos/2007 ATMOS http://drops.dagstuhl.de/opus/volltexte/2007/1171 db/conf/atmos/atmos2007.html#BrueraCDSF07