[It's](https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Promise.scala#L621-L648) [complicated](https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/concurrent/Scheduler.scala#L88-L209). I only understand it vaguely and at a high level.