docs: def _get_vertices_neighbours(nets):
      """Generate a listing of each vertex's immedate neighbours in an undirected
      interpretation of a graph.
  
      Returns
      -------
      {vertex: {vertex: weight, ...}), ...}
      """
      zero_fn = (lambda: 0)
      vertices_neighbours = defaultdict(lambda: defaultdict(zero_fn))
      for net in nets:
          if net.weight != 0:
              for sink in net.sinks:
                  vertices_neighbours[net.source][sink] += net.weight
                  vertices_neighbours[sink][net.source] += net.weight
      return vertices_neighbours