fn car (w :&World, x :Q) -> Ret<Q> {
    let y = {
        match &mut *x.clone().lock().unwrap() {
            Node::Cons(cell) =>
                Some(cell.car.clone()),
            _ =>
                None }};
    match y {
        Some(y) => Ok(y),
        None => error (w, "Not a cons", x.clone()) }}

// I don't like this solution because the error is not signal right
// where it happens. Suppose there would be more information to an error
// than just "Oh! Sorry.". What to do then? Define a new structure^Wenum
// for all the information?

// [Again: error() is a function dropping you into another REPL
// and _not_ a sentinel. ]