How did Yuan Tiangang, the first psychic in the Tang Dynasty, calculate that Wu Zetian would be emperor?

Yuan Tiangang's most famous physiognomy legend recorded in Old Tang Shu Shu is to visit Empress Wu Zetian. When Wu Zetian was still in infancy, Yuan Tiangang was surprised to see Wu Zetian's mother, Yang, and said, "A wife gives birth to a son!" Wu Zetian's mother sold her two sons Wu Hewu to Yuan Tiangang for face reading. But Yuan Tiangang said that he can be an official with Taoism, but he is only a risk-averse who can protect his family, and it should not be too expensive. Yang also called Wu Zetian's sister (later called Mrs. Han) out for Yuan Tiangang to take pictures. Yuan Tiangang said, "This woman is expensive and not good for her husband!" Finally, the nanny took out Wu Zetian's boy clothes. When Yuan Tiangang saw Wu Zetian in her infancy, she was surprised and said that she was a "phoenix-necked dragon, which was very expensive!" But he said regretfully, "It's a pity it's a man. If it is a woman, it is the master of the world! " ("If you are a woman, you can't spy, and then you are the master of the world!") ) "The historical process has been predicted by him.