System variable(s) used to unlock the menu links?

I think we need to clarify, the above solution (Math's original with my little addition) is not really  locking and unlocking the built-in menu. It simulates locking by covering up the menu with the overlay element the code creates--and the CSS classes make the menu items appear locked or unlocked--but the menu setting in the Storyline player has to be left as "Free" when you publish.

So, this technique is just a workaround. I would still like Articulate to give us the ability to actually lock or unlock specific menu items with a trigger, leaving whatever topics they missed in the pretest locked and required to view.* Clearly the coding is there because things are currently locked and unlocked, but only as the student views slides one at a time. The testing out feature I really want would require a trigger to unlock parts of the menu,

*Bear in mind, using this approach might affect the completion criteria your course is set to. Percent or number of slides viewed won't work if you're letting them test out of viewing certain slides.