merge(L, [], L).
merge([], L, L).
merge([H1|T1], [H2|T2], [H1|T]) :- H1 =< H2, merge(T1, [H2|T2], T).
merge([H1|T1], [H2|T2], [H2|T]) :- H2 < H1, merge([H1|T1], T2, T).