Macro myutil::alt

macro_rules! alt {
    ($condition: expr, $ops: block, $ops2: block) => { ... };
    ($condition: expr, $ops: block) => { ... };
    ($condition: expr, $ops: expr, $ops2: expr) => { ... };
    ($condition: expr, $ops: expr) => { ... };
}

高阶函数中提搞可读性.