DB transactions are now automatically nested if beginTransaction() is called when one is already in progress -- the queries aren't committed until the outermost commitTransaction() is called, which is handy for calling methods with their own transactions inside others when the entire process should be a transaction (e.g. Folder.erase() calling Item.erase() for the folder's descendents). Rollbacks are a little trickier -- currently, a rollback in a nested transaction flags the entire transaction for rollback when the outermost commit is reached, which for our purposes will probably be fine.
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