How do I enforce the existing of an element with a specific attribute value in the XML? For example: <events> <event type="system" desc="this is a system event"/> <event type="bla1" desc="this is bla1 event"/> <event type="bla2" desc="this is bla2 event"/> </events> I need a rule to make sure the event element with type attribute = 'system' exists (once). All other event elements are optional;