private static void AssertJoin(IEither <string, int> either)
 => Assert.That(either.Join(x => 1337, x => 1338), Is.EqualTo(1337));
 private static void AssertJoin(IEither <int, string> either)
 => Assert.That(either.Join(x => 1338, x => 1337), Is.EqualTo(1337));