Possible Clues:

• It's said to prove a rule
• The one that proves the rule
• Omission
• It proves the rule
• It doesn't conform to the general rule
• 'The . . . . . . . . . proves the rule'
• Means to test rule – I expect no changes
• Take it if you don't like it, but not as a rule
• Rejection? Not as a rule
• Something not following a rule