Abbas Edalat André Lieutier Dirk Pattinson A Computational Model for Multi-variable Differential Calculus. 505-519 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3441&spage=505 2005 conf/fossacs/2005 FoSSaCS db/conf/fossacs/fossacs2005.html#EdalatLP05