code: def prepare(self, left_tree, right_tree):
          
          # We don't want to diff comments:
          self._remove_comments(left_tree)
          self._remove_comments(right_tree)
  
          self.placeholderer.do_tree(left_tree)
          self.placeholderer.do_tree(right_tree)
docs: prepare() is run on the trees before diffing
  
          This is so the formatter can apply magic before diffing.