Eric Lippert's challenge - cleanererer

sealed class Dumper
{
    static public string Dump(Node root)
    {
        return Dump( root, "" );
    }
    static private string Dump(Node node, string indent)
    {
        return node.Text + "\n" + node.Children.Take(node.Children.Count - 1)
            .Aggregate(string.Empty,
                (results, child) => results + indent + "├─" + Dump(child, indent + "│ ") ) +
            ( node.Children.Count == 0 ? "" : indent + "└─" + Dump(node.Children.Last(), indent + "  " ) );
    }
}