Write a Prolog program, remove-nth(Before, After) that asserts the After list is the Before list with the removal of every n’th item from every list at all levels.

% Write a Prolog program, remove-nth(Before, After) that asserts the After list
% is the Before list with the removal of every n’th item from every list at all 
% levels.

/*delete a number in the list. */
delte(1,[_|T],T).
delte(P,[X|Y],[X|R]):-
 P1 is P-1,
 delte(P1,Y,R).

/* delete before and after. */
daltob(P,L,R):-
 P1 is P-1,
 delte(P1,L,R1),
 /* delete before. */
 delte(P,R1,R).
 /* delete after. */

% Output
Prolog Program to delete before and after of the nth element in the list.
% The above Prolog program was not working for boundary conditions when P is 1
% or when P is n(size of the list). Some changes are made in the code to make
% it work for boundary conditions as well. 
%         :::::::::::Code after above mentioned changes::::::::::::
% Write a Prolog program, remove-nth(Before, After) that asserts the After list
% is the Before list with the removal of every n’th item from every list at all 
% levels.

/*delete a number in the list. */
delte(1,[_|T],T).
delte(P,[X|Y],[X|R]):-
 P1 is P-1,
 delte(P1,Y,R).

/* delete before and after. */

% Its also working for first and last positions.
% Used nested if else conditions.

daltob(P,L,R):-
 length(L,L1),
 (
 P=:=1 ->
  P3 is P+1,
  delte(P3,L,R)
  ;
 P=:=L1 ->
  P3 is P-1,
  delte(P3,L,R)
  ;
  P1 is P-1,
  delte(P1,L,R1),
  /* delete before. */
  delte(P,R1,R)
  /* delete after. */
 ).

% Output
Prolog Program to delete before and after of the nth element in the list.