clientv3/concurrency/stm.go
func (ws writeSet) cmps(rev int64) []v3.Cmp {
	cmps := make([]v3.Cmp, 0, len(ws))
	for key := range ws {
		cmps = append(cmps, v3.Compare(v3.ModRevision(key), "<", rev))
	}
	return cmps
}
func (ws writeSet) cmps(rev int64) []v3.Cmp {
	cmps := make([]v3.Cmp, 0, len(ws))
	for key := range ws {
		cmps = append(cmps, v3.Compare(v3.ModRevision(key), "<", rev))
	}
	return cmps
}
// cmps returns a cmp list testing no writes have happened past rev