code: function undo(g) {
    g.eachEdge(function(e, s, t, a) {
      if (a.reversed) {
        delete a.reversed;
        g.delEdge(e);
        g.addEdge(e, t, s, a);
      }
    });
  }
docs: /*
  Given a graph that has had the acyclic operation applied, this function
  undoes that operation. More specifically, any edge with the `reversed`
  attribute is again reversed to restore the original direction of the edge.