[Yen]

I believe GP's point is that Magic: The Gathering was not intended to be a turing-complete set of rules and cards.

I.e., the game design was _intended_ to be entirely decidable. Indeed, certain game rules discuss what happens when events trigger each other in a loop - and if the loop is non-halting, the game ends in a draw. This rule is only meaningful if the halting problem for M:tG is decidable!

However, in creating enough "relatively simple" cards, most of which are of the form "when one thing happens, another thing then happens", the designers of M:tG have managed to _accidentally_ create a non-decidable system.

Sure, this particular example is contrived, but it has the implications that a non-contrived game state could find itself in an undecidable state! It is theoretically possible that a natural, tournament-level game could reach a state where it can't be decided if one player or the other wins, or if it's a draw. (Or, at minimum, deciding the outcome of the game might require sufficiently convincing a tournament official of your mathematical proof that the game in fact will halt , within some known finite time).

[794CD01]

The situation you describe isn't even that farfetched. There is a moderately popular deck called Four Horsemen that depends on repeating a set of steps to repeatedly shuffle your deck until the cards happen to be arranged in a sequence that allows you to win.

http://www.mtgsalvation.com/forums/the-game/legacy-type-1-5/...

[GuB-42]

"Shahrazad" had the potential to make impossibly long games, possibly even undecidable with the right combo. It has been used to force draws by stalling games.

For that reason, it is now banned in all tournament formats. In fact, it is the only "normal" card to receive this treatment.