In this photo provided by St. Louis University, Thomas B. Allen, author of Possessed: The True Story of an Exorcism, speaks during a panel discussion at St. Louis University in St. Louis, Tuesday, Oct. 29, 2013, about the month-long 1949 demon-purging ritual at the school’s former Alexian Brothers Hospital. The treatment of an unidentified suburban Washington, D.C., boy formed the basis for the 1971 novel by William Peter Blatty, The Exorcist, and the film of the same name two years later. (AP Photo/St. Louis University, Michelle Peltier)ST. LOUIS (AP) — Saint Louis College junior Zach Grummer-Strawn has never considered “The Exorcist,” the 1973 horror film thought to be some of the most interesting examples of unadulterated cinematic terror. He is handiest vaguely acquainted with the monthlong 1949 demon-purging ritual at his school on which the movie and William Peter Blatty’s 1971 novel were primarily based.