Christophe Dehlinger Jean-François Dufourd Formalizing the Trading Theorem for the Classification of Surfaces. 148-163 2002 conf/tphol/2002 TPHOLs http://link.springer.de/link/service/series/0558/bibs/2410/24100148.htm db/conf/tphol/tphol2002.html#DehlingerD02