public Guid ReadPackedGuid()
        {
            var mask = ReadByte();

            if (mask == 0)
                return new Guid(0);

            ulong res = 0;

            var i = 0;
            while (i < 8)
            {
                if ((mask & 1 << i) != 0)
                    res += (ulong)ReadByte() << (i * 8);

                i++;
            }

            return new Guid(res);
        }