code: 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);
  }
docs: /*
  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.