Jason Reed Extending Higher-Order Unification to Support Proof Irrelevance. 238-252 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2758&spage=238 2003 conf/tphol/2003 TPHOLs db/conf/tphol/tphol2003.html#Reed03