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) = 3*x + (-2)*y subtractIneq (2*y <=? 3*x ~ False) = -3*x + (-2)*y