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