prelude
名词：序曲，前奏曲; 开场戏，序幕; 前兆，预兆。
The Option
enum is so useful that it’s even included in the prelude; you don’t need to bring it into scope explicitly.Its variants are also included in the prelude: you can use Some and None directly without the Option:: prefix.
exhaustive
adj. 详尽的，彻底的
Matches in Rust are exhaustive: we must exhaust every last possibility in order for the code to be valid.
exhaust: v. 耗尽; 使精疲力竭; 详尽地讨论
Concise
adj. 简明的，简洁的
Concise Control Flow with if let