}
    return null
  }

  /**
   * Cancel all calls currently enqueued or executing. Includes calls executed both
   * [synchronously][Call.execute] and [asynchronously][Call.enqueue].
   */
  fun cancelAll() {
    this.withLock {
      for (call in readyAsyncCalls) {
        call.call.cancel()
      }
      for (call in runningAsyncCalls) {
        call.call.cancel()
      }