(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(x, e) ->  x
 f(e, x) -> x
 a(a(x)) -> x
 b(b(x)) -> x
 a(b(a(b(a(b(x)))))) -> x
)