&quot;Write tests for every behavior the system currently exhibits.&quot;<p>That&#x27;s often harder than just bashing your head against the problem. Retrofitting code bases with any sort of unit testing is almost always a challenge on its own terms... one much easier when one has at least an idea of how it&#x27;s working already. And that&#x27;s after I apply the principle of charity and downgrade &quot;every behavior&quot; to &quot;many behaviors of interest&quot;.<p>It&#x27;d be great if it works, but I suspect it would only work when it&#x27;s unnecessary anyhow because any code base amenable to that treatment wasn&#x27;t that hard of a problem in the first place.