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

    flatten([X|Xs],Ys) :- 
        flatten(X,Ys1), flatten(Xs,Ys2), append(Ys1,Ys2,Ys).
    flatten(X,[X]) :-
	constant(X), X \== [].
    flatten([],[]).


% Program 9.1a   Flattening a list with double recursion