I said, then you reevaluate the situation. When there are new requirements you do a quick estimate if you should add four new lines to the existing 20 or if it

I said, then you reevaluate the situation. When there are new requirements you do a quick estimate if you should add four new lines to the existing 20 or if it is worth to switch to an external library. 4 new lines to the 20, just add them to the core. But if this is regularly occurring that you have to add things, or requirements affecting this particular little parser that was supposed to be simple and static isn’t, then you should probably change your decision and use the library. But you do that only then. Because chances are that with your approach you are going to drag along a large generic library that you only use a tiny fraction of. And that also has costs. In particular if your immediate impulse always is to add another library instead of writing things yourself.