In the math around his head after discovering the vampires are near, Lucien is trying to find what where his chances of meeting the dryad —P(A)—, what were the chances of meeting TWO bosses one right after another — P(B|A)— and what are his chances of meeting both of them — P(A∩B)—. He concludes the later is zero — P(A∩B)=0—.... right before the vampire enters the bar