间接访问变量使得不可能分支和使用该变量，而无需重新绑定。匹配提供@印记就是出于这样的目的：
// A function `age` which returns a `u32`.
fn age() -> u32 {
15
}
fn main() {
println!("Tell me type of person you are");
match age() {
0 => println!("I'm not born yet I guess"),
// Could `match` 1 ... 12 directly but then what age
// would the child be? Instead, bind to `n` for the
// sequence of 1 .. 12. Now the age can be reported.
n @ 1 ... 12 => println!("I'm a child of age {:?}", n),
n @ 13 ... 19 => println!("I'm a teen of age {:?}", n),
// Nothing bound. Return the result.
n => println!("I'm an old person of age {:?}", n),
}
}
