docs: def _find_unchanged(old, new):
      """
      returns edges that are in both old and new
      """
      edges = []
      old_edges = [set(edge) for edge in old.edges()]
      new_edges = [set(edge) for edge in new.edges()]
      for old_edge in old_edges:
          if old_edge in new_edges:
              edges.append(set(old_edge))
      return edges