async Task Produce(ITargetBlock<string> queue, int howmuch)
{
    Random r = new Random();
    while (howmuch-- > 0)
    {
        await Task.Delay(1000 * r.Next(1, 3));
        var v = string.Format("automatic {0}", r.Next(1, 10));
        await queue.SendAsync(v);
    }
    queue.Complete();
}