trait ActorRef[-T] {
  def !(msg: T): Unit = ()
}

class Behavior[T]

object Behaviors {
  def receive[T](onMessage: T => Behavior[T]): Behavior[T] = ???
  def same[T]: Behavior[T] = ???
}

sealed trait Command

final case class Spawn[T](behavior: Behavior[T], replyTo: ActorRef[ActorRef[T]]) extends Command

object Test {
  def spawn[U](behavior: Behavior[U]): ActorRef[U] = ???

  def apply(): Behavior[Command] = Behaviors.receive { msg => 
    msg match {
      case Spawn(b: Behavior[t], replyTo) =>
        def sp(): ActorRef[t] = if (toString == "") sp() else spawn(b)
        replyTo ! sp()
        Behaviors.same
    }
  }
}