%Source: https://sites.google.com/site/prologsite/prolog-problems
%query:insert_at(g,g,g,f).
% P21 (*): Insert an element at a given position into a list
% The first element in the list is number 1.

% insert_at(X,L,K,R) :- X is inserted into the list L such that it
%    occupies position K. The result is the list R.
%    (element,list,integer,list) (?,?,+,?)

insert_at(X,L,K,R) :- remove_at(X,R,K,L).

remove_at(X,[X|Xs],1,Xs).
remove_at(X,[Y|Xs],K,[Y|Ys]) :- K > 1, 
   K1 is K - 1, remove_at(X,Xs,K1,Ys).