/*
Perform a DFS postorder traversal, labeling each node v with
its traversal order 'lim(v)' and the minimum traversal number
of any of its descendants 'low(v)'.  This provides an efficient
way to test whether u is an ancestor of v since
low(u) <= lim(v) <= lim(u) if and only if u is an ancestor.
function computeLowLim(tree) {
  var postOrderNum = 0;
  
  function dfs(n) {
    var children = tree.successors(n);
    var low = postOrderNum;
    for (var c in children) {
      var child = children[c];
      dfs(child);
      low = Math.min(low, tree.node(child).low);
    }
    tree.node(n).low = low;
    tree.node(n).lim = postOrderNum++;
  }

  dfs(tree.graph().root);
}