const add_SNo : set set set term + = add_SNo infix + 2281 2280 const minus_SNo : set set term - = minus_SNo const SNo : set prop var x:set var y:set var z:set hyp SNo x hyp SNo z hyp x + y = x + z hyp SNo - x hyp - x + x + y = y claim - x + x + z = z -> y = z