KeyOf[T] - literal keys of T
老牌厂商如汉王、掌阅都推出了自家的新产品，而其他领域的平台终端厂商如华为、微信读书也都有了自己的垂类产品，还有许多快速崛起的新兴厂牌，像文石、阅星瞳等便是其中之一。
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timeout: Seconds to wait before raising `asyncio.TimeoutError`. None means wait forever.
Commutativity: states can be merged in any order; A ∨ B = B ∨ A. If Alice and Bob exchange states, they can each merge the other’s state into their own and arrive at the same result.