(x + y) + z == (x :: Int) + (y + z)

0 + y == (y :: Int)

x + 0 == (x :: Int)

(xs ++ ys) ++ zs == (xs :: String) ++ (ys ++ zs)

[] ++ ys == (ys :: String)

xs ++ [] == (xs :: String)

(((f::Fun) . (g::Fun)) . (h::Fun)) (x::Int) == (f . (g . h)) x

(id . g) x == (g :: Fun) (x :: Int)

(f . id) x == (f :: Fun) (x :: Int)