% 1.16 (**):  Drop every N'th element from a list

% drop(L1,N,L2) :- L2 is obtained from L1 by dropping every N'th element.
%    (list,integer,list) (?,+,?)

drop(L1,N,L2) :- drop(L1,N,L2,N).

% drop(L1,N,L2,K) :- L2 is obtained from L1 by first copying K-1 elements
%    and then dropping an element and, from then on, dropping every
%    N'th element.
%    (list,integer,list,integer) (?,+,?,+)

drop([],_,[],_).
drop([_|Xs],N,Ys,1) :- drop(Xs,N,Ys,N).
drop([X|Xs],N,[X|Ys],K) :- K > 1, K1 is K - 1, drop(Xs,N,Ys,K1).