def start_tree(self, tree, filename):
        """Some fixers need to maintain tree-wide state.
        This method is called once, at the start of tree fix-up.

        tree - the root node of the tree to be processed.
        filename - the name of the file the tree came from.
        """
        self.used_names = tree.used_names
        self.set_filename(filename)
        self.numbers = itertools.count(1)
        self.first_log = True