Ah, now the shoe's on the other foot. Stephanie will savor the moment of superiority before answering.
"When you accuse a player of cheating you have to allude to how. Anything that violates the agreed-upon rules of the game is definitely cheating. Things that violate the presumed rules of the game… depends. If you're paranoid you can go around specifying that rock crushes scissors cuts paper covers rock every time you play, no one will hold it against you, but you can just say 'the game is Rock Paper Scissors' and that will do. It's not possible to redefine the outcome as 'rock tears paper, paper covers scissors, scissors stabs rock' midway through a best-of-three, even if you didn't clarify at the start."
She is speaking from experience, naturally. Everyone tries it at least once when they're little.
"You explained the rules of Idiot Poker as they differ, but there are still presumed rules like 'only you can bet using your own chips' and 'changing your call to a fold after showdown is not allowed'. No matter how fastidious you are about defining the rules of a game, there will still be some dumb edge cases that are covered by the presumed rules."
Again, this she knows from experience.
"There are also unwritten rules, which differ from presumed rules in that they're more ambiguous given the premise of the game. This is less of a problem for humans, since we can't use magic. Uh, if you imagine a chess game between two elves, and they didn't agree on a rule to not use magic, one of them could read the other one's mind and that may or may not be cheating. It would be a presumed rule in poker, that you don't know what your opponent's cards are, but chess doesn't have a presumption of secret information like that. The human equivalent would be… I don't know, maybe participating in a race after eating a magic berry that makes you run faster. Violating unwritten rules is usually not construed as cheating, so unwritten rules are often codified as written rules. If you subscribe to the 'collective cultural understanding' hypothesis of what constitutes cheating, it's possible that all the major chess tournaments specifically banning telepathy have made it so that mind-reading is more of an unwritten rule than a presumed rule for chess tournaments that don't ban it."
Human inability to use magic is a handicap on par with agreeing to only play paper, in Stephanie's opinion.