Who is the soothsayer in the story of the door god?

"In the thirteenth year of Tang Zhenguan, there was a teacher named Yuan Shoucheng in Chang 'an, who was a fortune teller. It is said that he can know yin and yang and break life and death. This man's background is not simple. He was the uncle of Yuan Tiangang, the supervisor of Qin Dynasty, and Yuan Tiangang was a famous astrologer in Tang Dynasty.