If you can't imagine your way to the cuplable code, try this:

Temporarily change `SymTree#symbol_=` to keep track of all modifications, rather than overrwrite the field. When you hit spot the tree with the null symbol later on, print out the list of places that called `symbol_=`. (You can add `Thread.currentThread.getStackTrace` to a buffer. Just make sure you have a big heap! You might also need to change `TreeCopier` to propagate your debug info.