Function rollback 

pub fn rollback(source: &Release, finalized: &BTreeMap<String, i16>) -> Rollback

Whether a cluster that finalized finalized may roll back to source.

It may when the source supports every finalized level. A feature the source does not name supports level 0 only, so finalizing such a feature above 0 also rules a rollback out. A feature that is not finalized is at level 0, which every release supports.