clientv3/concurrency/stm.go
func (rs readSet) cmps() []v3.Cmp {
	cmps := make([]v3.Cmp, 0, len(rs))
	for k, rk := range rs {
		cmps = append(cmps, isKeyCurrent(k, rk))
	}
	return cmps
}
func (rs readSet) cmps() []v3.Cmp {
	cmps := make([]v3.Cmp, 0, len(rs))
	for k, rk := range rs {
		cmps = append(cmps, isKeyCurrent(k, rk))
	}
	return cmps
}
// cmps guards the txn from updates to read set