func KeyMissing(key string) clientv3.Cmp {
	return clientv3.Compare(clientv3.Version(key), "=", 0)
}
func KeyMissing(key string) clientv3.Cmp {
	return clientv3.Compare(clientv3.Version(key), "=", 0)
}
// KeyMissing returns a comparison operation that evaluates to true iff the
// given key does not exist.