Code review comment for lp:~sylvain-pineau/checkbox/checkbox-ng-sa

My only -1 is on the `._export_results`. I really believe that talking to plainbox internals behind SessionAssistant could be avoided. (See inline comment).

Building tree (create_simple_tree) smells bad, the nested coprehension in the loop.
But after understanding how the code works I guess that's the cleanest way (reading-wise) of doing it, and the list of categories in the root node *should* be small.

Super minor thing: rev 4121 ‘aa’ typo in commit msg