Testing for a subset

Source: The Art of Prolog

Program source code: list_member_4.pl

/*
   members(Xs,Ys) :- Each element of the list Xs is an element of the list Ys.
*/
     members([X|Xs],Ys) :- member(X,Ys), members(Xs,Ys).
     members([],Ys).
 
%  Program 7.6    Testing for a subset