code: @SuppressWarnings("unchecked")
    protected <T> Completes<T> completes() {
      if (completes == null) {
        throw new IllegalStateException("Completes is not available for this protocol behavior.");
      }
      return (Completes<T>) completes;
    }
docs: Answers the {@code Completes<T>} instance for this {@code Actor}, or {@code null} if the behavior of the currently
  delivered {@code Message} does not answer a {@code Completes<T>}.
  @param <T> the protocol type
  @return {@code Completes<T>}