code: private Mailbox allocateMailbox(final Definition definition, final Address address, final Mailbox maybeMailbox) {
      final Mailbox mailbox = maybeMailbox != null ?
              maybeMailbox : ActorFactory.actorMailbox(this, address, definition);
      return mailbox;
    }
docs: Answers a Mailbox for an Actor. If maybeMailbox is allocated answer it; otherwise
  answer a newly allocated Mailbox. (INTERNAL ONLY)
  @param definition the Definition of the newly created Actor
  @param address the Address allocated to the Actor
  @param maybeMailbox the possible Mailbox
  @return Mailbox