SFQBWL6PHSX7MQVOSQW4ZO35SLXNHN6562IWYCN4UKWAEVE6AQNQC while let Some(elt) = stack.pop() {
// `to_self` tells whether we've already marked a vertex alive// along the path from `vertex0`. This can happen if the only// vertices alive are from change `change`.while let Some((elt, mut to_self)) = stack.pop() {