Just like there is a difference between interface and implementation in OOP languages, here 'theory' is in the heads of the original builders of the software, a

Just like there is a difference between interface and implementation in OOP languages, here 'theory' is in the heads of the original builders of the software, and the 'implementation' is the code written in particular languages. It is easy to debug and fix when one has that 'theory' in one's head.