two more tries (boxed list and reference to boxed list).

both don't work as desired, boxed list moves the value and the reference
to it has the same error as all the other reference things.

rust/pointers.rs

    //     }
    // }
    pub fn cons_with_box<A>(x: A, xs: Box<List<A>>) -> List<A> {
        match *xs {
            Nil => Cons(x, xs),
            _   => Cons(x, xs)
        }
    }
    // pub fn cons_with_referenced_box<A>(x: A, xs: &Box<List<A>>) -> List<A> {
    //     match **xs {
    //         Nil     => Cons(x, box Nil),
    //         ref l   => Cons(x, box *l) // "cannot move ...", again
    //     }
    // }
    pub fn run() {
        println!("\nlist_with_box:");
        println!("l = {}", l);
        println!("cons(3, l)  = {:?}", cons(3, l));
        println!("cons(4, l2) = {:?}", cons(4, l2));
        println!("");
        // println!("cons_with_ref(3, &l) = {:?}", cons_with_ref(3, &l));
        //                                                        // ^    "use of moved value: `l`"
        let lb: Box<List<int>> = box cons_with_box(1, box cons_with_box(2, box Nil));
        println!("lb = {:?}", lb);
        println!("cons_with_box(3, lb) = {:?}", cons_with_box(3, lb));
        // println!("cons_with_box(4, lb) = {:?}", cons_with_box(4, lb));
        //                                                       // ^     "use of moved value: `l`"
    }
}