I've generated the graphs for the full 8.5 source code and I think it contains precious information about how feap is implemented.
I did not share it because it violates the terms of use of feap.
the graph generation is straightforward:
1) install doxygen on your system
2) put the configuration file (attached) at ../ver85
3) run in terminal: doxygen [name of configuration file]
(change configurations if needed)
if you want I can show you the results I got so far with this method