When he did notice it, it was too late; the cab refused to obey the orders he at once punched into it.  Caxton realized bitterly that he had let himself be trapped by a means no professional hoodlum would fall for: his call had been traced, his cab identified, its idiot robot pilot placed under orders of an over-riding police frequency - and the cab itself was being used to arrest him and fetch him in, all most privately and with no fuss,