Swift提供了Case Pattern Initializer用来初始化一个变量，如下可以判断a是否在10~20之间。
if case 10 … 20 = a {
print(“a is in 10 ~ 20”)
}
可参考Swift语言标准条件语句语法：
condition → expression | availability-condition | case-condition | optional-binding-condition
case-condition → case pattern initializer
Refer to: Statements | Documentation (swift.org)
