Semigroup sum of disjunction

Example.scala

  /**
   * Sums up values inside disjunction, if both are left or right. Returns first left otherwise.
   * {{{
   * \/-(v1) +++ \/-(v2) → \/-(v1 + v2)
   * \/-(v1) +++ -\/(v2) → -\/(v2)
   * -\/(v1) +++ \/-(v2) → -\/(v1)
   * -\/(v1) +++ -\/(v2) → -\/(v1 + v2)
   * }}}
   */
  def +++(x: => A \/ B)(implicit M1: Semigroup[B], M2: Semigroup[A]): A \/ B =
    this match {
      case -\/(a1) => x match {
        case -\/(a2) => -\/(M2.append(a1, a2))
        case \/-(_) => this
      }
      case \/-(b1) => x match {
        case b2 @ -\/(_) => b2
        case \/-(b2) => \/-(M1.append(b1, b2))
      }
    }