"Cell with two traversal modes:
 -[[rest]] for storage
 -[[next]]/[[previous]] for stable iteration
 This allows us to use the same cell object in two different lists which
 have the same elements but different iteration order."
class LinkedCell<Element>(Element car, Cell<Element>? cdr, previous) 
        extends Cell<Element>(car, cdr) {
    shared variable LinkedCell<Element>? next = null;
    shared variable LinkedCell<Element>? previous;

class LinkedCellIterator<Element>(iter) 
        satisfies Iterator<Element> {
    variable LinkedCell<Element>? iter;
    shared actual Element|Finished next() {
        if (exists iter = iter) {
            this.iter = iter.next;
            return iter.element;
        return finished;