code: func (s *Simulation) UpNodeIDs() (ids []enode.ID) {
  	nodes := s.Net.GetNodes()
  	for _, node := range nodes {
  		if node.Up() {
  			ids = append(ids, node.ID())
  		}
  	}
  	return ids
  }
docs: // UpNodeIDs returns NodeIDs for nodes that are up in the network.