'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.'

def start_tree(self, tree, filename):

    self.used_names = tree.used_names
    self.set_filename(filename)
    self.numbers = itertools.count(1)
    self.first_log = True
