* canceled, even if this plan is canceled.
     */
    fun retry(): Plan?
  }

  /**
   * What to do once a plan has executed.
   *
   * If [nextPlan] is not-null, another attempt should be made by following it. If [throwable] is
   * non-null, it should be reported to the user should all further attempts fail.
   *
   * The two values are independent: results can contain both (recoverable error), neither