>there is a set [...with] mechanics
Welcome to the definition of a Turing Complete system.
Also the game in question is not "innocuous." The author admits to creating an artificial scenario under which a Turing Machine is possible.
Welcome to the definition of a Turing Complete system.
Also the game in question is not "innocuous." The author admits to creating an artificial scenario under which a Turing Machine is possible.
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).
http://www.mtgsalvation.com/forums/the-game/legacy-type-1-5/...
For that reason, it is now banned in all tournament formats. In fact, it is the only "normal" card to receive this treatment.