Tournament rule 505.2.a

Loops must be fully deterministic; they may not rely on probability, decision trees, or conditional actions. Example:

• 505.2.a.1 Legal Loop: With enough Energy available, playing Grim Apothecary to a battlefield then using its trigger to return…
• 505.2.a.2 Illegal Loop: A card reads "Recycle 3 cards from your trash: Ready a unit." A player with enough Energy available…

Official Tournament Rules