public static double Average(this IEnumerable<int> source) {
            if (source == null) throw Error.ArgumentNull("source");
            long sum = 0;
            long count = 0;
            checked {
                foreach (int v in source) {
                    sum += v;
                    count++;
                }
            }
            if (count > 0) return (double)sum / count;
            throw Error.NoElements();
        }