/*
   flatten(Xs,Ys) :- Ys is a list of the elements of Xs.
*/
     flatten(Xs,Ys) :- flatten(Xs,[],Ys).

     flatten([X|Xs],S,Ys) :- 
		list(X), flatten(X,[Xs|S],Ys).
     flatten([X|Xs],S,[X|Ys]) :- 
		constant(X), X \== [], flatten(Xs,S,Ys).
     flatten([],[X|S],Ys) :- 
		flatten(X,S,Ys).
     flatten([],[],[]).

     list([X|Xs]).

% Program 9.1b   Flattening a list using a stack