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The "FromReader" functions were indeed added for testing purposes, but not for testing this package. This function makes it possible to have deterministic UUID's returned when validating a piece of software that uses this uuid package. I think NewV7 would be the most common way to get a V7 uuid. Using NewV7FromReader would only be used by software that needs this ability. That software would probably do something like:

import "crypto/rand"

var rander = rand.Read
...
    u := NewV7FromReader(rander)

and then in their validation tests they would change what rander points to.