(from AG01 3.36) (VAR x y) (RULES minus(x,0) -> x minus(s(x),s(y)) -> minus(x,y) f(0) -> s(0) f(s(x)) -> minus(s(x),g(f(x))) g(0) -> 0 g(s(x)) -> minus(s(x),f(g(x))) )