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)