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