fn compose<X, Y, Z>(a: Box<Fn(Y) -> Z>, b: Box<Fn(X) -> Y>) -> Box<Fn(X) -> Z>
    where X: 'static,
          Y: 'static,
          Z: 'static
{
    Box::new(move |c| a(b(c)))
}

fn main() {
    let composed = compose(Box::new(move |num| num - 1), Box::new(move |num| num * 2));

    println!("{}", composed(10));
    //=> 19
}