ABSTRACT
Imagine that you are responsible for controlling access to Chicago’s United Center during a Chicago Bulls–Los Angeles Lakers game. The United Center has multiple entrances; you lock some of them, but you can’t lock them all. The fans, teams, concessionaires, and press all need to get in and out. You post guards at the open doors to check credentials—tickets, press passes, and so on—but you know that some unauthorized people may still slip by. So you tell the credential-checking guards to be on the lookout for suspicious behavior, and you hire some other guards whose primary task is to monitor for and investigate such behavior.
