intersection(nil,Y,nil)

intersection(X!L,M,X!N) :-
  mem(X,M) & intersection(L,M,N)

intersection(X!L,M,N) :-
  ~mem(X,M) & intersection(L,M,N)

  mem(X,X!L)
  mem(X,Y!L) :- mem(X,L)

union(nil,Y,Y)

union(X!L,M,N) :-
  mem(X,M) & union(L,M,N)

union(X!L,M,X!N) :-
  ~mem(X,M) & union(L,M,N)