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What should you do if the player makes a mistake?
Added 5 minutes ago
You're wrong. Launching it through multiplayer and simply setting the AI as an opponent will yield the same result. Therefore, it cannot be determined this way.
There is also a third way. Define a test function that will 100% work differently in single-player and multiplayer. Run it and, based on the results, determine whether it's single-player or multiplayer.