private static bool Dswap(int[] a, int i, int j, int d) {
            bool r = false;

            if (i == j) return true;

            if (a[i] >= j - d && a[i] <= j + d &&
                a[j] >= i - d && a[j] <= i + d) {
                r = true;
                int t = a[i];
                a[i] = a[j];
                a[j] = t;

                if (a[i] == i) throw new Exception("Was here!");
                if (a[j] == j) throw new Exception("Was here!");
            }

            return r;
        }