Ronald W. Ferguson Joseph L. Bokor Rudolph L. Mappus IV Adam Feldman Maintaining Spatial Relations in an Incremental Diagrammatic Reasoner. 136-150 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2825&spage=136 2003 conf/cosit/2003 COSIT db/conf/cosit/cosit2003.html#FergusonBMF03