Doc was developed using the InterViews UI toolkit. I believe that doc source is part of the InterViews distribution. Doc was used to typeset Paul’s thesis. (Paul Calder was my lecturer at Flinders University)
If you look at the InterViews code you might be surprised. It was developed before modern C++ existed. For example, there are no templates. And there are no comments in the code.
To my understanding, Lexi never existed. It was created as an example for the book by GoF.