A loop is a form of tournament shortcut that involves detailing a sequence of actions to be repeated and then performing a number of iterations of that sequence. The loop actions must be identical in each iteration and cannot include conditional actions (“If this, then that”.)
If no players are involved in maintaining the loop, each player in turn order chooses a number of iterations to perform before they will take an action to break the loop or that they wish to take no action. If all players choose to take no action, the game is a draw. Otherwise, the game advances through the lowest number of iterations chosen and the player who chose that number takes an action to break the loop.
If one player is involved in maintaining the loop, they choose a number of iterations. The other players, in turn order, agree to that number or announce a lower number after which they intend to intervene. The game advances through the lowest number of iterations chosen and the player who chose that number receives priority.
If two or more players are involved in maintaining a loop within a turn, each player in turn order chooses a number of iterations to perform. The game advances through the lowest number of iterations chosen and the player who chose that number receives priority.
Loops may span multiple turns if a game state is not meaningfully changing. Note that drawing cards other than the ones being used to sustain the loop is a meaningful change. If two or more players are involved in maintaining a loop across turns, each player chooses a number of iterations to perform, or announces their intent to continue indefinitely. If all players choose to continue indefinitely, the game is a draw. Otherwise, the game advances through the lowest number of iterations chosen and the player who chose that number receives priority at the point they stop taking an action to sustain the loop.
A player intervening during a loop may specify that one iteration of the loop is only partly performed in order to be able to take action at the appropriate point. If they do, the final iteration is only performed up to the chosen point.
Non-deterministic loops (loops that rely on decision trees, probability or mathematical convergence) may not be shortcut. A player attempting to execute a nondeterministic loop must stop if at any point during the process a previous game state (or one identical in all relevant ways) is reached again. This happens most often in loops that involve shuffling a library.
Some loops are sustained by choices rather than actions. In these cases, the rules above may be applied, with the player making a different choice rather than ceasing to take an action. The game moves to the point where the player makes that choice. If the choice involves hidden information, a judge may be needed to determine whether any choice is available that will not continue the loop.
The judge is the final arbiter of what constitutes a loop. A player may not ‘opt-out’ of shortcutting a loop, nor may they make irrelevant changes between iterations in an attempt to make it appear as though there is no loop. Once a loop has been shortcut, it may not be restarted until the game has changed in a relevant way. Proposing loops as an effort to use up time on the clock is Stalling.