subtracting n = iso (subtract n) ((+n) subtracting n = from (adding n)
subtracting n = iso (subtract n) ((+n) subtracting n = re (adding n)
subtractIneq (2*y <=? 3*x ~ True) = (-2*y + 3*x) subtractIneq (2*y <=? 3*x ~ False) = (-3*x + (-1) + 2*y)