Integrating traceability concerns in VDD process

Issue raised by @konnov, @josef-widder and @adizere.

- We have to figure out traceability. Designing the code will be an iterative process. How we keep the code and TLA+ in sync during change is a challenge I think.
- Perhaps we can introduce explicit versioning of code & spec artefacts. And enforce versioning as a part of the VDD process.
- We also need a common mechanism to refer to the individual parts of the properties (requirements), pre- and post-conditions, etc. These concepts have been formalized and standardised in the aircraft industry. However, it is hard to find an open and comprehensive description of the processes, as their certification processes have been commercialised. (Reading the standards themselves does not help much.) I just googled for a high-level overview. This document (
https://www.cadence.com/content/dam/cadence-www/global/en_US/documents/solutions/aerospace-and-defense/do-254-explained-wp.pdf) at least gives some ideas.