% -*-prolog-*-

% represent a set as a list with no duplicate elements.

% union(+Set1, +Set2, -Result)  Result is the union of sets
% Set1 and Set2

union([], S2, S2).

union([First | Rest], S2, U):-
        member(First, S2), 
        !,              % if member succeeds, but we later fail back
                        % to here, fail union; don't redo member or union.
        union(Rest, S2, U).

union([First | Rest], S2, [First | U]):-
        union(Rest, S2, U).

% member(Element, List)

member(First, [First | _]).

member(E, [_ | Rest]):-
        member(E, Rest).