func WithPrefetch(keys ...string) stmOption {
	return func(so *stmOptions) { so.prefetch = append(so.prefetch, keys...) }
}
func WithPrefetch(keys ...string) stmOption {
	return func(so *stmOptions) { so.prefetch = append(so.prefetch, keys...) }
}
// WithPrefetch is a hint to prefetch a list of keys before trying to apply.
// If an STM transaction will unconditionally fetch a set of keys, prefetching
// those keys will save the round-trip cost from requesting each key one by one
// with Get().