/**   A Stepper is something that has methods for stepping 
 *    forward or backward.
 */
public interface Stepper {

   /**   move forward one (and only one) step.  
    *    If it is not possible (e.g., the stepper is hitting a wall), 
    *    then stepForward() throws a
    *    CannotStepException.
    */
   public void stepForward() 
      throws CannotStepException;

   /**   move backward.  (and only one) step.  
    *    If it is not possible (e.g., the stepper is hitting a wall), 
    *    then stepBackward() throws a
    *    CannotStepException.
    */
   public void stepBackward()
      throws CannotStepException;
}