def all[T, C[X] <: Traversable[X]](fs: C[Future[T]])(implicit cbf: CanBuildFrom[C[T], T, C[T]], ec: ExecutionContext): Future[C[T]] = {
 val b = cbf()
 val p = Promise[b.type]()
 p.success(b)
 fs.foldLeft(p.future) { (acc, f) =>
   for { x <- f; b <- acc } yield b += x
 } map { _.result() }
}