docs: def _preprocess_nodes_for_pydot(nodes_with_data):
      """throw away all node attributes, except for 'label'"""
      for (node_id, attrs) in nodes_with_data:
          if 'label' in attrs:
              yield (quote_for_pydot(node_id),
                     {'label': quote_for_pydot(attrs['label'])})
          else:
              yield (quote_for_pydot(node_id), {})