serializable semigroup instances

  implicit def tuple2Semigroup[A, B](implicit A: Semigroup[A], B: Semigroup[B]): Semigroup[(A, B)] = instance[(A, B)] {
    case ((la, lb), (ra, rb)) => (A.append(la, ra), B.append(lb, rb))
  }

  /** Make an associative binary function into an __Serializable__ instance. */
  def instance[A](f: (A, => A) => A): Semigroup[A] = SerializableSemigroup(f)


  private case class SerializableSemigroup[A](f: (A, => A) => A) extends Semigroup[A] {
    private val self: Semigroup[A] = this

    override def append(lhs: A, rhs: => A): A = f(lhs, rhs)

    override val semigroupSyntax: SSemigroupSyntax[A] = new SSemigroupSyntax[A] with Serializable {
      def F: Semigroup[A] = self
    }
  }