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Destroy the Evidence
(mtgzone.com)
Post your custom card designs, card alters, custom sets, and anything else Magic-related designed by you or other community members!
This is a neat idea. I think it would function as an Alchemy card, where the software could keep track of what had created the token -- in fact, I wouldn't be surprised if it already does that. But yeah, for paper play, it would be a nightmare. Was the Mite you're targeting created by this Mirrex, or by that other Mirrex that got Field-of-Ruined a couple of turns ago?
Yeah you're right, it's just too much overhead. The examples below create the tracking requirement when those cards are played, this would require that you always track every token from the start of the game forever.
Could definitely work in Alchemy though.