(VAR x y z) (RULES f(x,f(y,z)) -> f(f(x,y),z) f(x,i(x)) -> e f(i(x), x) -> e f(e, x) -> x f(x,e) -> x f(a, f(a, f(a, f(a, f(a, f(a, a)))))) -> e f(b, f(b, f(b, f(b, f(b, f(b, b)))))) -> e f(a, f(b, f(a, b))) -> e f(i(a), f(b, f(i(a), f(b, f(i(a), b))))) -> e )