https://github.com/ReactiveX/RxSwift/blob/master/Documentation/MathBehindRx.md
https://github.com/ReactiveX/RxSwift/blob/12cccb171ad9038251af6883807f0290c1d75a5b/Documentation/GettingStarted.md
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原文：https://www.cnblogs.com/feng9exe/p/9092177.html
