Andrea Calì Diego Calvanese Maurizio Lenzerini Semistructured data schemas with expressive constraints. 3-16 KRDB2000/P003.pdf 2000 conf/krdb/2000 KRDB http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS/Vol-29/01-cali-calv-lenz.ps db/conf/krdb/krdb2000.html#CaliCL00