In contrast to the effortless ease with which human beings control their limbs, the design of controllers for robotic manipulator arms is a detailed, meticulous business. Motors controlling the arms need to be started and stopped at just the right moment so that the performance demanded by the user may be achieved at the end of a complicated manoeuvre. And yet, the same user wishes to express the task for the robot in the simplest possible terms without reference to the minute details of control sequences that his task demands. It is the design of such inter- faces between man and machine...