03.Answer

def sequence[A](fas: List[F[A]]): F[List[A]] =
  traverse(fas)(identity)

def traverse[A, B](as: List[A])(f: A => F[B]): F[List[B]] =
  as.foldRight(unit(List.empty[B]))((a, acc) => f(a).map2(acc)(_ :: _))