Here's how an exception proving (testing) the rule works:
A swimming pool has a notice on the diving board that reads "Not for use by children under 8."
Anyone can therefore use the diving board with the exception of under-8s.
Along comes one such exception in the form of a 7 year-old kid. Lifeguard sees him and says "stay off the board, son!"
Now just extend that to quasars and you're on a winner.