clientv3/op.go
func OpTxn(cmps []Cmp, thenOps []Op, elseOps []Op) Op {
	return Op{t: tTxn, cmps: cmps, thenOps: thenOps, elseOps: elseOps}
}
func OpTxn(cmps []Cmp, thenOps []Op, elseOps []Op) Op {
	return Op{t: tTxn, cmps: cmps, thenOps: thenOps, elseOps: elseOps}
}
// OpTxn returns "txn" operation based on given transaction conditions.