let mut unused_changes = Vec::new();
            'outer: for x in btree::rev_iter(&self.txn, &c, None)? {
                let (_, p) = x?;
                for chan in self.iter_channels("").map_err(|e| e.0)? {
                    let (name, chan) = chan.map_err(|e| e.0)?;
                    if name.as_str() == name0 {
                        continue
                    }
                    let chan = chan.read().unwrap();
                    if self.channel_has_state(&chan.states, &p.b).map_err(|e| e.0)?.is_some() {
                        break 'outer
                    }
                    if self.get_changeset(&chan.changes, &p.a).map_err(|e| e.0)?.is_some() {
                        continue 'outer
                    }
                }
                unused_changes.push(p.a);
            }
            for ch in unused_changes.iter() {
                while btree::del(&mut self.txn, &mut self.dep, ch, None)? {}
                while btree::del(&mut self.txn, &mut self.revdep, ch, None)? {}
            }