code: function debugOrdering(g) {
    var layerMatrix = util.buildLayerMatrix(g);
  
    var h = new Graph({ compound: true, multigraph: true }).setGraph({});
  
    _.forEach(g.nodes(), function(v) {
      h.setNode(v, { label: v });
      h.setParent(v, "layer" + g.node(v).rank);
    });
  
    _.forEach(g.edges(), function(e) {
      h.setEdge(e.v, e.w, {}, e.name);
    });
  
    _.forEach(layerMatrix, function(layer, i) {
      var layerV = "layer" + i;
      h.setNode(layerV, { rank: "same" });
      _.reduce(layer, function(u, v) {
        h.setEdge(u, v, { style: "invis" });
        return v;
      });
    });
  
    return h;
  }
docs: /* istanbul ignore next