const SNo : set prop const Empty : set axiom SNo_0: SNo Empty const add_SNo : set set set term + = add_SNo infix + 2281 2280 axiom add_SNo_0L: !x:set.SNo x -> Empty + x = x axiom add_SNo_com: !x:set.!y:set.SNo x -> SNo y -> x + y = y + x claim !x:set.SNo x -> x + Empty = x