const SNo : set prop const add_SNo : set set set term + = add_SNo infix + 2281 2280 const minus_SNo : set set term - = minus_SNo const Empty : set axiom add_SNo_minus_SNo_linv: !x:set.SNo x -> - x + x = Empty axiom add_SNo_com: !x:set.!y:set.SNo x -> SNo y -> x + y = y + x var x:set hyp SNo x claim SNo - x -> x + - x = Empty