lastb.scala

def lastB[A, B](implicit S:Strategy): Pipe2[Task, A, B, (A, B)] = (as, bs) =>
  bs.uncons1.flatMap {
    case None => Stream.empty
    case Some((init, newBs)) =>
      as.either(newBs).mapAccumulate(init) {
        case (b, Left(a)) => (b, Some(a))
        case (_, Right(b)) => (b, None)
      }.collect { case (b, Some(a)) => (a, b) }
  }