/*
Replace edge e with edge f in the tree, recalculating the tree root,
the nodes' low and lim properties and the edges' cut values.
function exchange(graph, tree, e, f) {
  tree.delEdge(e);
  var source = graph.source(f);
  var target = graph.target(f);

  // Redirect edges so that target is the root of its subtree.
  function redirect(v) {
    var edges = tree.inEdges(v);
    for (var i in edges) {
      var e = edges[i];
      var u = tree.source(e);
      var value = tree.edge(e);
      redirect(u);
      tree.delEdge(e);
      value.reversed = !value.reversed;
      tree.addEdge(e, v, u, value);
    }
  }

  redirect(target);

  var root = source;
  var edges = tree.inEdges(root);
  while (edges.length > 0) {
    root = tree.source(edges[0]);
    edges = tree.inEdges(root);
  }

  tree.graph().root = root;

  tree.addEdge(null, source, target, {cutValue: 0});

  initCutValues(graph, tree);

  adjustRanks(graph, tree);
}