* occur after a call is canceled. Typically cancellation takes effect when an expensive I/O
   * operation is required.
   *
   * This is invoked at most once, even if [Call.cancel] is invoked multiple times. It may be
   * invoked at any point in a call's life, including before [callStart] and after [callEnd].
   */
  open fun canceled(call: Call) {
  }

  /**
   * Invoked when a call fails due to cache rules.