func (s *Subscr) Unsubscribe(oid packet.OptionId) {
errs.Check(oid.Valid(), "unsubscribing from invalid option", oid)
delete(s.subscriptions, oid)
}
func (s *Subscr) Subscribe(oid packet.OptionId) {
errs.Check(oid.Valid(), "subscribing to invalid option", oid)
s.subscriptions[oid] = struct{}{}
s.autoSubscribe = false
}