Volker Gruhn Ursula Wellen Process Landscaping: Modelling Distributed Processes and Proving Properties of Distributed Process Models. 103-125 2001 conf/apn/2001u Unifying Petri Nets http://link.springer.de/link/service/series/0558/bibs/2128/21280103.htm db/conf/apn/upn2001.html#GruhnW01