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