code: synchronized void setDeadLetters(final DeadLetters deadLetters) {
      if (deadLetters != null && this.deadLetters != null) {
        deadLetters.stop();
        throw new IllegalStateException("Dead letters already exists.");
      }
  
      this.deadLetters = deadLetters;
    }
docs: Sets the {@code DeadLetters} as the default for this {@code World}. (INTERNAL ONLY)
  @param deadLetters the {@code DeadLetters} to register as the default