// use std::collections::hash_map::Entry;

            cache.insert(*vertex, Some(alive.clone()));

                cache.insert(*vertex, Some(alive.clone()));

    // Each element of the stack is a pair of (1) an edge pointing to
    // the position we want to visit next and (2) optionally, a set of
    // alive vertices found until now.
    // Every time we find an alive vertex, we add it to the sets of
    // each element on the path to that vertex.

        debug!("pop {:?} {:?}", elt, alive);

            cache.insert(vertex, (Some(alive), files_));

                cache.insert(vertex, (Some(alive), files_));

        } else {
            if !visited.insert(elt.dest()) {
                continue;
            }
            stack.push((elt, Some((HashSet::new(), HashSet::new()))));
        };

        }

            }
            if visited.insert(elt.dest()) {
                stack.push((elt, Some((HashSet::new(), HashSet::new()))));

            continue;
        }
        if !visited.insert(elt.dest()) {

        stack.push((elt, Some((HashSet::new(), HashSet::new()))));

                debug!("just push");