It's easy to replace two kinds of functions. One is thoroughly specified and covered by extensive input-output pair test-cases. The other is completely unspec

It's easy to replace two kinds of functions. One is thoroughly specified and covered by extensive input-output pair test-cases. The other is completely unspecified, and not possible to pin down with test cases due to constantly churning. (The users quickly learned not to rely on any specific outputs, so the churn doesn't break anyone.) Everything in between is hard to replace to varying degrees, because it's reliable enough that users can depend on specific behaviors, yet what users dare rely on is not always obvious from specification or covered with tests.