/*
   selects(Xs,Ys) :- The list Xs is a subset of the list Ys.
*/
     selects([X|Xs],Ys) :- select(X,Ys,Ys1), selects(Xs,Ys1).
     selects([],Ys).

	select(X,[X|Xs],Xs).
	select(X,[Y|Ys],[Y|Zs]) :- select(X,Ys,Zs).

%  Program 7.7    Testing for a subset