Paul W. P. J. Grefen Samuil Angelov Three-Level Process Specification for Dynamic Service Outsourcing: From Petri Nets to ebXML and WFPDL. 264-280 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2472&spage=264 2003 conf/dfg/2003 Petri Net Technology for Communication-Based Systems db/conf/dfg/petri2003.html#GrefenA03