const In : set set prop term iIn = In infix iIn 2000 2000 const SNoLev : set set const SNo : set prop const SNoS_ : set set const add_SNo : set set set term + = add_SNo infix + 2281 2280 var x:set var y:set var z:set hyp SNo x hyp !w:set.w iIn SNoS_ (SNoLev x) -> w + y = y + w hyp SNo z hyp SNoLev z iIn SNoLev x claim z iIn SNoS_ (SNoLev x) -> z + y = y + z