def par_fib_(n: Int): Long = {
    def fibHelper(n: Int): Future[Long] =
      if (n<=46) Future{ fib(n) } else {
        val f1: Future[Long] = fibHelper(n-1)
        val f2: Future[Long] = fibHelper(n-2)

//        Await.result(f1, Duration.Inf) + Await.result(f2, Duration.Inf)
        for { nf1 <- f1
              nf2 <- f2 } yield nf1 + nf2

        f1.flatMap( nf1 => f2.map ( nf2 => nf1 + nf2 ) )
      }

    // NOTE: the 'join' form, converting Future[Long] to Long
    Await.result(fibHelper(n), Duration.Inf)
  }