Herman Geuvers Randy Pollack Freek Wiedijk Jan Zwanenburg A Constructive Algebraic Hierarchy in Coq. 271-286 2002 34 J. Symb. Comput. 4 http://dx.doi.org/10.1006/jsco.2002.0552 db/journals/jsc/jsc34.html#GeuversPWZ02