Penny Anderson Frank Pfenning Verifying Uniqueness in a Logical Framework. 18-33 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3223&spage=18 2004 conf/tphol/2004 TPHOLs db/conf/tphol/tphol2004.html#AndersonP04