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fn main() {
    println!("{}", (1, (2, (4, Empty()))).get(2));
}
 
struct Empty ();
 
trait Cons<T> {
    type Tail: Cons<T>;
    
    fn get_head(&self) -> &T;
    fn get_tail(&self) -> &Self::Tail;
    
    fn get(&self, index: usize) -> &T {
        match index {
            0 => self.get_head(),
            x => self.get_tail().get(x - 1)
        }
    }
}
 
impl<T, C:Cons<T>> Cons<T> for (T, C) {
    type Tail = C;
    
    fn get_head(&self) -> &T {
        &self.0
    }
    fn get_tail(&self) -> &Self::Tail {
        &self.1
    }
}
 
impl<T> Cons<T> for Empty {
    type Tail = Empty;
    
    fn get_head(&self) -> &T {
        panic!()
    }
    fn get_tail(&self) -> &Self::Tail {
        panic!()
    }
}