#[error("Cannot dropped a borrowed channel: {:?}", c)]
    ChannelRc { c: String },

    fn drop_channel(&mut self, name: &str) -> Result<bool, Self::GraphError> {
        let name = SmallString::from_str(name);
        self.open_channels.borrow_mut().remove(&name);
        debug!("drop_channel {:?}", name);
        let result = btree::del(&mut self.txn, &mut self.channels, &name, None)?;
        debug!("/drop_channel {:?}: {:?}", name, result);
        Ok(result)

    fn drop_channel(&mut self, name0: &str) -> Result<bool, Self::GraphError> {
        let name = SmallString::from_str(name0);
        let channel = if let Some(channel) = self.open_channels.borrow_mut().remove(&name) {
            let channel = Rc::try_unwrap(channel.r)
                .map_err(|_| SanakirjaError::ChannelRc { c: name0.to_string() })?
                .into_inner();
            Some((
                channel.graph,
                channel.changes,
                channel.revchanges,
                channel.states,
            ))
        } else if let Some((name_, chan)) = btree::get(&self.txn, &self.channels, &name, None)? {
            if name_ == name.as_ref() {
                Some((
                    Db::from_page(chan.0[0].into()),
                    Db::from_page(chan.0[1].into()),
                    UDb::from_page(chan.0[2].into()),
                    UDb::from_page(chan.0[3].into()),
                ))
            } else {
                None
            }
        } else {
            None
        };
        if let Some((a, b, c, d)) = channel {
            btree::drop(&mut self.txn, a)?;
            btree::drop(&mut self.txn, b)?;
            btree::drop(&mut self.txn, c)?;
            btree::drop(&mut self.txn, d)?;
        }
        Ok(btree::del(&mut self.txn, &mut self.channels, &name, None)?)