Logical connective invented by the logician Arthur Prior as a ‘runabout inference ticket’. Its rules are two: first, from P you may infer P tonk Q, and second from P tonk Q you may infer Q. Put together these imply that from P you may infer Q, which is incoherent. The example puts pressure on philosophies which think that the laws of logic are no more than conventions governing the stipulated behaviour of defined connectives.