debug!(target: "drop_channel", "drop channel {:?}", name0);

                for chan in self.iter_channels("").map_err(|e| e.0)? {
                    let (name, chan) = chan.map_err(|e| e.0)?;

                debug!(target: "drop_channel", "testing unused change: {:?}", p);
                let mut it0 = self.iter_channels("").map_err(|e| e.0)?;
                let it1 = self.open_channels.lock();
                let mut it1 = it1.iter();
                loop {
                    let (name, chan) = if let Some(chan) = it0.next() {
                        chan.map_err(|e| e.0)?
                    } else if let Some((name, chan)) = it1.next() {
                        (name.as_ref(), chan.clone())
                    } else {
                        break
                    };
                    debug!(target: "drop_channel", "channel: {:?}", name);

                        // This other channel is in the same state as
                        // our dropped channel is, so all subsequent
                        // patches are in use.

                        // This channel has a patch, move on.

                debug!(target: "drop_channel", "actually unused: {:?}", p);

                    btree::del(&mut self.txn, &mut self.revdep, &k, Some(&v))?;

                    debug!(target: "drop_channel", "deleting from revdep: {:?} {:?}", k, v);
                    btree::del(&mut self.txn, &mut self.dep, &k, Some(&v))?;