The basis of the idea evolved over several years and is a bit disjointed. The actual implementation was an epiphany. In the early 1990s, I was working as a systems engineer at (then named) Spar Aerospace on the specifications for the robotic arm (Canadarm 2) to be mounted on the International Space Station. Part of my job was the maintenance of traceability of requirements between the hierarchy of documents. This responsibility was accentuated when one of the levels of the hierarchy was deleted and the traceability was significantly altered. This work was done manually. I guess this process was then imprinted on my brain.
In my next (and current) position I worked on a project that involved the hierarchy of messages on a multi-line LCD. Continuous requirement changes necessitated the development of a versatile approach to implement the modified message structure quickly and easily.
While writing my book "Excel by Example: A Microsoft Excel Cookbook for Electronics Engineers," I investigated and wrote about the auditing features of Excel, but I took the functions at face value-a support tool to aid with the debugging of a worksheet.
The project involving the LCD messaging was ongoing, and I wanted to develop a better method of composing the four lines of a screen within the context of a hierarchy of screens. It was at this point that I gor the idea (probably in the shower-all y good ideas start there!) that the linkage could be visually displayed using the audit feature of Excel. I don't remember whether the association with the tracebility of requirements was the progenitor of the messaging approach or vice versa. It was just one of those things where the human brain combined several disparate ideas together to generate an elegant solution.