Info needed to rewrite gdb print-object

"I'd like to learn more about these ObjectiveC containers, though. Can they be nesting? Can I programatically inquire on them like how many objects, etc...Basically, maybe there's a way I can write my own version of "print-object" that is more flexible..."
The above questions are from the author of Seer who is adding Objective-C support to his Seer gdb debugger frontend. I am also interested to learn more on this.
Here is a screenshot of Seer debugging Objective-C:

The ticket where Objective-C support is discussed is here: https://github.com/epasveer/seer
Thanks,Mike