code: func (g *Graph) SelectTopNodePtrs(maxNodes int, visualMode bool) NodePtrSet {
  	set := make(NodePtrSet)
  	for _, node := range g.selectTopNodes(maxNodes, visualMode) {
  		set[node] = true
  	}
  	return set
  }
docs: // SelectTopNodePtrs returns a set of the top maxNodes *Node in a graph.