len(0, []).
len(N, [_|B]) :- len(M, B), N is M+1.

head([X|_]) :- print(X).
tail([_|X]) :- print(X).

memberx(X,[X|_]).
memberx(X,[_|B]) :- memberx(X,B).

unionx([A|X],Y,[A|Z]) :- not(memberx(A,Y)), unionx(X,Y,Z).
unionx([A|X],Y,Z) :- memberx(A,Y), unionx(X,Y,Z).
unionx([],X,X).

intersectionx([A|X],Y,Z) :- not(memberx(A,Y)), intersectionx(X,Y,Z).
intersectionx([A|X],Y,[A|Z]) :- memberx(A,Y), intersectionx(X,Y,Z).
intersectionx([],_,[]).

differencex([A|X],Y,Z) :- memberx(A,Y), differencex(X,Y,Z).
differencex([A|X],Y,[A|Z]) :- not(memberx(A,Y)), differencex(X,Y,Z).
differencex([],_,[]).