enum List {
    // Cons: Tuple struct that wraps an element and a pointer to the next node
    Cons(u32, Box<List>),
    // Nil: A node that signifies the end of the linked list
    Nil,
}

fn car(l : &List) -> u32 {
    match *l {
        Nil => 420,
        Cons(n, _) => n
    }
}

fn cdr(l: &List) -> &List {
    match *l {
        Nil => &Nil,
        Cons(_,ref tail) => tail
    }
}