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. [Jargon File] (1995-02-20)