fn rplacd (w :&World, x :Q, y :Q) -> Ret<Q> {
    match &mut *x.clone().lock().unwrap() {
        Node::Cons(cell) =>
        { cell.cdr = y; Ok(x) },
        _ => error (w, "Not a cons", x.clone()) }}
//           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//           Here the whole Q is still locked