It had been the year after their 20th birthdays. Christmas Eve. Just like today. He thinks that it's the reason why that memory is so strong, so persistent. So intense.   "Nana," Yixing calls out teasingly. "Let's go Christmas shopping?"  He stands near the door, voice muffled by his thick and warm scarf around his neck and cold chafed hands rubbing with each other to make it warm.