You could be better off looking at a "condition" than a "validator". A condition hides and blocks a transition if the conditions are not met. You can easily place conditions like "user must be in role", "user must be in group" and/or "user must be Dave" on transitions.
Validators happen after a user clicks on the transition, so they may get annoyed that they were offered a transition they could not use, or lose data they were trying to add during the transition. You can use validators to do it, but I would avoid it for those reasons. Validators are more for checking the input that a user does during a transition than checking if they can do it.
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