Panos Vassiliadis Alkis Simitsis Spiros Skiadopoulos Modeling ETL activities as graphs. 52-61 2002 conf/dmdw/2002 DMDW http://SunSITE.Informatik.RWTH-Aachen.de/Publications/CEUR-WS/Vol-58/simitsis.pdf db/conf/dmdw/dmdw2002.html#VassiliadisSS02