func (d send) check(c *gc.C, in chan params.RelationUnitsChange, out chan hook.Info) {
	ruc := params.RelationUnitsChange{Changed: map[string]params.UnitSettings{}}
	for name, version := range d.changed {
		ruc.Changed[name] = params.UnitSettings{Version: version}
	}
	for _, name := range d.departed {
		ruc.Departed = append(ruc.Departed, name)
	}
	in <- ruc
}

func (q *AliveHookQueue) loop(initial *State) {
	defer q.tomb.Done()
	defer watcher.Stop(q.w, &q.tomb)

	// Consume initial event, and reconcile with initial state, by inserting
	// a new RelationUnitsChange before the initial event, which schedules
	// every missing unit for immediate departure before anything else happens
	// (apart from a single potential required post-joined changed event).
	ch1, ok := <-q.w.Changes()
	if !ok {
		q.tomb.Kill(watcher.MustErr(q.w))
		return
	}
	if len(ch1.Departed) != 0 {
		panic("AliveHookQueue must be started with a fresh RelationUnitsWatcher")
	}
	q.changedPending = initial.ChangedPending
	ch0 := params.RelationUnitsChange{}
	for unit, version := range initial.Members {
		q.info[unit] = &unitInfo{
			unit:    unit,
			version: version,
			joined:  true,
		}
		if _, found := ch1.Changed[unit]; !found {
			ch0.Departed = append(ch0.Departed, unit)
		}
	}
	q.update(ch0)
	q.update(ch1)

	var next hook.Info
	var out chan<- hook.Info
	for {
		if q.empty() {
			out = nil
		} else {
			out = q.out
			next = q.next()
		}
		select {
		case <-q.tomb.Dying():
			return
		case ch, ok := <-q.w.Changes():
			if !ok {
				q.tomb.Kill(watcher.MustErr(q.w))
				return
			}
			q.update(ch)
		case out <- next:
			q.pop()
		}
	}
}

func setRelationUnitChangeVersion(changes *params.RelationUnitsChange, key string, revno int64) {
	name := unitNameFromScopeKey(key)
	settings := params.UnitSettings{Version: revno}
	if changes.Changed == nil {
		changes.Changed = map[string]params.UnitSettings{}
	}
	changes.Changed[name] = settings
}

// mergeScope starts and stops settings watches on the units entering and
// leaving the scope in the supplied RelationScopeChange event, and applies
// the expressed changes to the supplied RelationUnitsChange event.
func (w *relationUnitsWatcher) mergeScope(changes *params.RelationUnitsChange, c *RelationScopeChange) error {
	for _, name := range c.Entered {
		key := w.sw.prefix + name
		revno, err := w.mergeSettings(changes, key)
		if err != nil {
			return err
		}
		changes.Departed = remove(changes.Departed, name)
		w.st.watcher.Watch(w.st.settings.Name, key, revno, w.updates)
		w.watching.Add(key)
	}
	for _, name := range c.Left {
		key := w.sw.prefix + name
		changes.Departed = append(changes.Departed, name)
		if changes.Changed != nil {
			delete(changes.Changed, name)
		}
		w.st.watcher.Unwatch(w.st.settings.Name, key, w.updates)
		w.watching.Remove(key)
	}
	return nil
}