PrologP12A

domains
elem=integer
list=elem*

predicates
union(list, list, list)
member(elem, list)

clauses
union([A|B], C, D) :- member(A, C), union(B, C, D).
union([A|B], C, [A|D]) :- union(B, C, D).
union([], A, A).

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

goal
union([1, 2, 3], [1, 2, 3, 4, 5], L),
write(L).