I wish I could remember who wrote the essay with the idea of tests as investment in protecting functionality. When after a bit of experimentation or iteration y

I wish I could remember who wrote the essay with the idea of tests as investment in protecting functionality. When after a bit of experimentation or iteration you think you have figured out more or less one part of how your software should behave, then you want to protect that result. It is worth investing in writing and maintaining a test to make sure you don't accidentally break this functionality. Functionality based on a set of initial specs and a hazy understanding of the actual problem you are trying to solve might on the other hand might not be worth investing in protecting.