code: synchronized void setPrivateRoot(final Stoppable privateRoot) {
      if (privateRoot != null && this.privateRoot != null) {
        privateRoot.stop();
        throw new IllegalStateException("Private root already exists.");
      }
  
      this.privateRoot = privateRoot;
    }
docs: Sets the {@code PrivateRootActor} instances as a {@code Stoppable}. (INTERNAL ONLY)
  @param privateRoot the {@code Stoppable} protocol backed by the {@code PrivateRootActor}