% P09 (**):  Pack consecutive duplicates of list elements into sublists.

% pack(L1,L2) :- the list L2 is obtained from the list L1 by packing
%    repeated occurrences of elements into separate sublists.
%    (list,list) (+,?)

pack([],[]).
pack([X|Xs],[Z|Zs]) :- transfer(X,Xs,Ys,Z), pack(Ys,Zs).

% transfer(X,Xs,Ys,Z) Ys is the list that remains from the list Xs
%    when all leading copies of X are removed and transfered to Z

transfer(X,[],[],[X]).
transfer(X,[Y|Ys],[Y|Ys],[X]) :- X \= Y.
transfer(X,[X|Xs],Ys,[X|Zs]) :- transfer(X,Xs,Ys,Zs).