for &hash in change.dependencies.iter() {
        if let Hash::None = hash {
            continue;
        }
        if let Some(int) = txn.get_internal(hash) {
            if txn.get_changeset(&channel.changes, int, None).is_some() {
                continue;
            }
        }
        return Err((Error::DependencyMissing { hash }).into());
    }