Self-test 6, Task 1, Solutions

Adding to position/3

The requirement for the position to be zero when the list is empty can be satisfied by adding a fact to the position/3 procedure: 
     /* ************************************************ */
     /*                                                  */
     /*   position/3                                     */
     /*      Arg 1:  Counter: integer                    */
     /*      Arg 2:  Elem to be found                    */
     /*      Arg 3:  List with Elem in                   */
     /*   Summary: is true if List contains Elem at the  */
     /*            nth position.                         */
     /*   Author: P J Hancox                             */
     /*   Date:   21 October 1994                        */
     /*                                                  */
     /* ************************************************ */

     % 1
     position(1, Elem, [Elem|_]).
     % 2 
     position(0, _, []).
     % 3
     position(Cnt, Elem, [_|Tail]) :-
          Cnt1 is Cnt - 1,
          position(Cnt1, Elem, Tail).