# [Haskell], 35 bytes

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    a#b|all(`elem`[1..99])[a,b,a*b]=a*b

[Try it online!][TIO-j3hw2gle] Example usage: `3#4` returns `12`, while `3.5#4`, `3#400` and `3#40` all yield the error `non-exhaustive pattern in function #`.

[Haskell]: https://www.haskell.org/
[TIO-j3hw2gle]: https://tio.run/##y0gszk7Nyfn/P1E5qSYxJ0cjITUnNTch2lBPz9IyVjM6USdJJ1ErKdYWSPzPTczMU7BVKCjKzCtRUFEwVjb5DwA "Haskell – Try It Online"