code: 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
docs: 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.