[eclipse 13]: S `:: {}.. {1,2,3,5}, S1 `:: {1} .. {1,2,3,4}, all_union([S, S1],{1,2,3,4,5}). S = S{{5} .. {1, 2, 3, 5}} S1 = S1{{1, 4} .. {1, 2, 3, 4}} Delayed goals: ground_union(S{{5} .. {1, 2, 3, 5}}, S1{{1, 4} .. {1, 2, 3, 4}}, {1, 2, 3, 4, 5}) yes.