into(task(S1, D1), task(S2, D2)) :-
  {S2 =< S1, S1 + D1 = SD1, S2 + D2 = SD2, SD1 =< SD2}.

noOverlap(_, []).
noOverlap(task(S1, D1), [task(S2, D2)|Tasks]) :-
  (
    {S1 + D1 = SD1, SD1 < S2}
  ;
    {S2 + D2 = SD2, SD2 < S1}
  ),
  noOverlap(task(S1, D1), Tasks).

schedule([], _).
schedule([Task|Tasks], Supertask) :-
  into(Task, Supertask),
  noOverlap(Task, Tasks),
  schedule(Tasks, Supertask).

test1(S) :-
  schedule([task(0, 1), task(3, 1), task(S, 1)], task(0, 6)).
test2(S) :-
  schedule([task(0, 28), task(70, 40), task(S, 35)], task(0, 240)).