// 演示用刚加的 either 库
pub fn demo_either(flag : Bool) -> String {
  let e : @either.Either[Int, String] = if flag {
    @either.Either::Left(42)
  } else {
    @either.Either::Right("hello")
  }
  match e {
    Left(v) => "Left:\{v}"
    Right(s) => "Right:\{s}"
  }
}