When we run our suites of automated tests, we do it to reduce risk. Specifically, to reduce the risk that we may have broken some code somewhere without knowing it.

When we refactor one or more lines of code, we run the risk of breaking not only the code we refactored, but any code that directly or indirectly depends on it.

To reduce the risk of refactoring, then (and refactoring is all about reducing risk), we need to retest any code that could possibly have...