I seek advice for where to find the answer to the question: Is there a (what is the) philosophy of "exceptions", as in "exceptions to the rule", and even more pointedly "exceptions that prove the rule"?
I.E. Is it legit for a "rule" or a definition to own a collection of "exceptions", or is that rule/definition just broken/inadequate?
E.G. If I were trying to come up with a definition of something, and had one except for its exceptions (that prove the rule of course!

can I declare victory and come home? Or, do I need to go back to the drawing board till I get rid of all exceptions?
I am not looking for opinions, but rather references to books/articles/URLs (or at least which subcategory of philosophy that the topic likely lies in).