@Synchronized
  fun queuedCalls(): List<Call> = readyAsyncCalls.map { it.call }.unmodifiable()

  /** Returns a snapshot of the calls currently being executed. */
  @Synchronized
  fun runningCalls(): List<Call> = (runningSyncCalls + runningAsyncCalls.map { it.call }).unmodifiable()

  @Synchronized
  fun queuedCallsCount(): Int = readyAsyncCalls.size

  @Synchronized