member(X, [X|Rest]).
member(X, [Head|Rest]) :- member(X, Rest).


not-member(Element, []).
not-member(Element, [First | Left-over]) :- 
         _1 .= princ-to-string(First),
         _2 .= princ-to-string(Element),
         string/=(_1, _2),
         not-member(Element, Left-over).