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What was it in reference to?

The old joke that mathematicians can't distinguish a donut from a coffee mug (because of topological "equivalence"/homeomorphism) [0]

To a topologist, a mug and a donut are the same shape (can be deformed to a torus without changing topological properties)

There's no way it wasn't, I laughed when I saw it

Could be a reference to Dunkin' Donuts