How to use the graphlib.Digraph function in graphlib

function sortLayerSubgraph(g, sg, cg, weights) {
  cg = cg ? cg.filterNodes(nodesFromList(g.children(sg))) : new Digraph();

  var nodeData = {};
  g.children(sg).forEach(function(u) {
    if (g.children(u).length) {
      nodeData[u] = sortLayerSubgraph(g, u, cg, weights);
      nodeData[u].firstSG = u;
      nodeData[u].lastSG = u;
    } else {
      var ws = weights[u];
      nodeData[u] = {
        degree: ws.length,
        barycenter: util.sum(ws) / ws.length,
        order: g.node(u).order,
        orderCount: 1,
        list: [u]
      };

function feasibleTree(g) {
  var remaining = new Set(g.nodes()),
      tree = new Digraph();

  if (remaining.size() === 1) {
    var root = g.nodes()[0];
    tree.addNode(root, {});
    tree.graph({ root: root });
    return tree;
  }

  function addTightEdges(v) {
    var continueToScan = true;
    g.predecessors(v).forEach(function(u) {
      if (remaining.has(u) && !slack(g, u, v)) {
        if (remaining.has(v)) {
          tree.addNode(v, {});
          remaining.remove(v);
          tree.graph({ root: v });

function feasibleTree(g) {
  var remaining = new Set(g.nodes()),
      tree = new Digraph();

  if (remaining.size() === 1) {
    var root = g.nodes()[0];
    tree.addNode(root, {});
    tree.graph({ root: root });
    return tree;
  }

  function addTightEdges(v) {
    var continueToScan = true;
    g.predecessors(v).forEach(function(u) {
      if (remaining.has(u) && !slack(g, u, v)) {
        if (remaining.has(v)) {
          tree.addNode(v, {});
          remaining.remove(v);
          tree.graph({ root: v });

function feasibleTree(g) {
  var remaining = new Set(g.nodes()),
      tree = new Digraph();

  if (remaining.size() === 1) {
    var root = g.nodes()[0];
    tree.addNode(root, {});
    tree.graph({ root: root });
    return tree;
  }

  function addTightEdges(v) {
    var continueToScan = true;
    g.predecessors(v).forEach(function(u) {
      if (remaining.has(u) && !slack(g, u, v)) {
        if (remaining.has(v)) {
          tree.addNode(v, {});
          remaining.remove(v);
          tree.graph({ root: v });

Graph.prototype.asDirected = function() {
  var g = new Digraph();
  this.eachNode(function(u, value) { g.addNode(u, value); });
  this.eachEdge(function(e, u, v, value) {
    g.addEdge(null, u, v, value);
    g.addEdge(null, v, u, value);
  });
  return g;
};

function mergeNodeData(g, lhs, rhs) {
  var cg = mergeDigraphs(lhs.constraintGraph, rhs.constraintGraph);

  if (lhs.lastSG !== undefined && rhs.firstSG !== undefined) {
    if (cg === undefined) {
      cg = new Digraph();
    }
    if (!cg.hasNode(lhs.lastSG)) { cg.addNode(lhs.lastSG); }
    cg.addNode(rhs.firstSG);
    cg.addEdge(null, lhs.lastSG, rhs.firstSG);
  }

  return {
    degree: lhs.degree + rhs.degree,
    barycenter: (lhs.barycenter * lhs.degree + rhs.barycenter * rhs.degree) /
                (lhs.degree + rhs.degree),
    order: (lhs.order * lhs.orderCount + rhs.order * rhs.orderCount) /
           (lhs.orderCount + rhs.orderCount),
    orderCount: lhs.orderCount + rhs.orderCount,
    list: lhs.list.concat(rhs.list),
    firstSG: lhs.firstSG !== undefined ? lhs.firstSG : rhs.firstSG,
    lastSG: rhs.lastSG !== undefined ? rhs.lastSG : lhs.lastSG,

function feasibleTree(g) {
  var remaining = new Set(g.nodes()),
      tree = new Digraph();

  if (remaining.size() === 1) {
    var root = g.nodes()[0];
    tree.addNode(root, {});
    tree.graph({ root: root });
    return tree;
  }

  function addTightEdges(v) {
    var continueToScan = true;
    g.predecessors(v).forEach(function(u) {
      if (remaining.has(u) && !slack(g, u, v)) {
        if (remaining.has(v)) {
          tree.addNode(v, {});
          remaining.remove(v);
          tree.graph({ root: v });

Graph.prototype.asDirected = function() {
  var g = new Digraph();
  this.eachNode(function(u, value) { g.addNode(u, value); });
  this.eachEdge(function(e, u, v, value) {
    g.addEdge(null, u, v, value);
    g.addEdge(null, v, u, value);
  });
  return g;
};