title: How to instantiate a materializer for AkkaStreams when I have a reference to the typed actor system?
body: <p>The code below does not compile, it says that ActorMaterializer is missing an implicit ActorRefFactory. How should I provide one?</p>
  
  <pre><code>val guardian: Behavior[Done] = Behaviors.setup(_ =&gt; {
    Behaviors.receiveMessage{
      case Done =&gt; Behaviors.stopped
    }
  })
  implicit val sys = ActorSystem(guardian, "sys")
  implicit val materializer: Materializer = ActorMaterializer()
  </code></pre>