/** The coproduct of two functors annihilates their product. */
  implicit def coproductProductZap[F[_], FF[_], G[_], GG[_]](implicit d1: Zap[FF, F], d2: Zap[GG, G]):
    Zap[({ type λ[α] = (FF[α], GG[α]) })#λ, ({ type λ[α] = (F[α] \/ G[α]) })#λ] =
      new Zap[({ type λ[α] = (FF[α], GG[α]) })#λ, ({ type λ[α] = (F[α] \/ G[α]) })#λ] {
        def zapWith[A, B, C](a: (FF[A], GG[A]), b: (F[B] \/ G[B]))(f: (A, B) => C) =
          b match {
            case -\/(fb) => d1.zapWith(a._1, fb)(f)
            case \/-(gb) => d2.zapWith(a._2, gb)(f)
          }
      }