public void Tokens_CategoriesEmpty ()
{
IEnumerable<int> s = new[]{1, 2, 3};
Func<int, int, int> a = (x, y) => x+y;
Func<int, Tuple<int, int>> rs = x => Tuple.Create (x, 0);
Func<int, int, bool>[] cats = new Func<int, int, bool>[0];
IEnumerable<int> r = s.Tokens (0, a, rs, cats);
}