Used to complete a proof in technical books when one doesn't mind a handwave, or to avoid one entirely. The complete phrase is: "The proof [or "the rest"] is left as an exercise for the reader." This comment *has* occasionally been attached to unsolved research problems by authors possessed of either an evil sense of humour or a vast faith in the capabilities of their audiences.
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Nearby terms: execution « executive « Executive Systems Programming Oriented Language « exercise, left as an » exhaustive testing » existential quantifier » exit