Dr. Neal Kassell was standing by the door to the operating room when then-Sen. Joe Biden was wheeled in for a second aneurysm operation in May 1988.Biden reached up from the gurney and grabbed the neurosurgeon’s arm.“He looked me in the eye and said, ‘Doc, do a good job, because someday I’m going to be president,’” Kassell told The Daily Beast on Saturday, after his former patient’s prediction had come true.Biden rode the gurney on into the OR at Walter Reed National Military Medical Center in Washington, D.C., and Kassell followed once Biden had been anesthetized for his second microsurgical craniotomy.The first operation had been that February after a scan had located two aneurysms at the base of Biden’s brain, the larger to the right, the smaller to the left. Kassell and a fellow neurosurgeon, Dr. Eugene George, decided to begin with the larger one. It was liable to burst at any moment, with possibly fatal results.Biden had withdrawn from the 1987 race for the Democratic nomination ...