Hans Jürgen Schneider Changing Labels in the Double-Pushout Approach Can Be Treated Categorically. 134-149 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3393&spage=134 2005 conf/birthday/2005ehrig Formal Methods in Software and Systems Modeling db/conf/birthday/ehrig2005.html#Schneider05