Function required_leaf_removals 

pub fn required_leaf_removals(
    conn: &impl DbQuery,
    old: &GroupMembership,
    new: &GroupMembership,
    diff_removals: &HashSet<Vec<u8>>,
    leaf_owners: &HashMap<Vec<u8>, String>,
) -> Result<HashSet<Vec<u8>>, IdentityDependencyError>

Required removals include leafed identity removals and stale leaves of every inbox whose membership entry advances or disappears.