Self-test 6, Task 3, Solutions

insert_before_pos/4

This is based on replace_at_pos/4, with the major change being in the terminating condition: 
     /* ************************************************ */
     /*                                                  */
     /*   insert_before_pos/4                            */
     /*      Arg 1:  Counter: integer                    */
     /*      Arg 2:  Elem to be added                    */
     /*      Arg 3:  List1                               */
     /*      Arg 3:  List2 is List1 with Elem added      */
     /*   Summary: is true if List2 is List1 with Elem   */
     /*            before the nth position.              */
     /*   Author: P J Hancox                             */
     /*   Date:   21 October 1994                        */
     /*                                                  */
     /* ************************************************ */

     % 1 terminating condition
     insert_before_pos(1, Elem, [Head|Tail], [Elem, Head|Tail]).
     % 2
     insert_before_pos(Pos, Elem, [Head|Tail1], [Head|Tail2]) :-
          Pos1 is Pos - 1,
          insert_before_pos(Pos1, Elem, Tail1, Tail2).