IN Arthur C. Clarke's short story Quarantine, a higher extra-terrestrial intelligence blows up planet earth, because every computer probe they sent in became hopelessly infected, and, soon, dysfunctional. "'They encountered a—problem—that couldn't be fully analysed within the lifetime of the Universe. Though it involved only six operators, they became totally obsessed by it.' 'How is that possible?' 'We do not know: we must never know. But if those six operators are ever re-discovered, all rational computing will end.'" The last line of the story reveals the identities of the six operators: king, queen, bishop, knight, rook, pawn.