% P20 (*): Remove the K'th element from a list.
% The first element in the list is number 1.
% remove_at(X,L,K,R) :- X is the K'th element of the list L; R is the
% list that remains when the K'th element is removed from L.
% (element,list,integer,list) (?,?,+,?)
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).