clientv3/concurrency/stm.go
func NewSTM(c *v3.Client, apply func(STM) error, so ...stmOption) (*v3.TxnResponse, error) {
	opts := &stmOptions{ctx: c.Ctx()}
	for _, f := range so {
		f(opts)
	}
	if len(opts.prefetch) != 0 {
		f := apply
		apply = func(s STM) error {
			s.Get(opts.prefetch...)
			return f(s)
		}
	}
	return runSTM(mkSTM(c, opts), apply)
}
func NewSTM(c *v3.Client, apply func(STM) error, so ...stmOption) (*v3.TxnResponse, error) {
	opts := &stmOptions{ctx: c.Ctx()}
	for _, f := range so {
		f(opts)
	}
	if len(opts.prefetch) != 0 {
		f := apply
		apply = func(s STM) error {
			s.Get(opts.prefetch...)
			return f(s)
		}
	}
	return runSTM(mkSTM(c, opts), apply)
}
// NewSTM initiates a new STM instance, using serializable snapshot isolation by default.