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   * this initiates service shutdown and returns immediately. If the service is {@linkplain
   * State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been started nor
   * stopped. If the service has already been stopped, this method returns immediately without
   * taking action.
   *
   * @return this
   * @since 15.0
   */
  @CanIgnoreReturnValue
  Service stopAsync();

  /**

  /**
   * An enum representing the state of a node during DFS. {@code PENDING} means that the node is on
   * the stack of the DFS, while {@code COMPLETE} means that the node and all its successors have
   * been already explored. Any node that has not been explored will not have a state at all.
   */
  private enum NodeVisitState {
    PENDING,
    COMPLETE
  }

  /**
   * An enum representing the state of a node during DFS. {@code PENDING} means that the node is on
   * the stack of the DFS, while {@code COMPLETE} means that the node and all its successors have
   * been already explored. Any node that has not been explored will not have a state at all.
   */
  private enum NodeVisitState {
    PENDING,
    COMPLETE
  }

  }

  /**
   * Posts an event to all registered subscribers. This method will return successfully after the
   * event has been posted to all subscribers, and regardless of any exceptions thrown by
   * subscribers.
   *
   * <p>If no subscribers have been subscribed for {@code event}'s class, and {@code event} is not
   * already a {@link DeadEvent}, it will be wrapped in a DeadEvent and reposted.
   *