Reader Question: What's the most complex data structure you've ever run into or had to program against?

My personal one was the "Mortgage Industry Standards Maintenance Organization" (you can download it if you're bored) which is a monster DTD that includes just about everything a real estate/mortgage or financial company could want.  In any case, creating "fake" data [for a customer prototype using BizTalk] for this thing was tough, and showing how each different partner organization could use part of this data in their application as part of an orchestrated process was tougher.

Here's another one (although I haven't written code for this), the Visual Studio 2005 has a way to export your IDE settings into a *.vssettings file. So I did this with C# Express and I got a nice 400K size text file. Ouch! To be fair, most of the size appears to be Base64 encoded data which bloats the file size quite a bit.

Back to the question: What's the most complex data structure you've ever run into or had to program against?