My opponent cast a doomsday during the current iteration of the arena powered cube, normally I should not know which cards are the 5 they have put back into their library or the order of them, however if they choose any cards which were in their graveyard the way the animation exiles the cards shows clearly any in the graveyard which are chosen. At first I thought it was an animation bug only, but the cards were fully revealed in their library as well after the resolution of doomsday. Obviously you can narrow it down if you have full information of their deck, cross referencing what has been exiled with cards which would have been in their graveyard, however I should still not have the order they were put into their library in fully revealed to me.