The closure of a set S, which is S*, is defined as S0 ∪ S1 ∪ S2 ∪ ...
So if S contained the English alphabet, S* = {empty string} ∪ {a,b,c,..} ∪ {aa, ab, ac, ... , ba, bb, bc, ... } ∪ {three character strings} ∪ ...
What is A* if A = {10}? I'm not sure how closure works for numbers because you can't just concatenate numbers like you can with strings.