Jean-Christophe Filliâtre Pierre Letouzey Functors for Proofs and Programs. 370-384 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2986&spage=370 2004 conf/esop/2004 ESOP db/conf/esop/esop2004.html#FilliatreL04