public static void AddExt(uint[] xx, uint[] yy, uint[] zz)
        {
            uint c = Nat224.AddExt(xx, yy, zz);

            if (c != 0 || (zz[13] == PExt13 && Nat224.GteExt(zz, PExt)))
            {
                Nat224.SubExt(zz, PExt, zz);
            }
        }
        public static void SubtractExt(uint[] xx, uint[] yy, uint[] zz)
        {
            int c = Nat224.SubExt(xx, yy, zz);

            if (c != 0)
            {
                Nat224.AddExt(zz, PExt, zz);
            }
        }