sitemap with security trimming

i have a sitemap with trimming feature based on membership roles. the site map looks like this and it is working.
<?xml version="1.0" encoding="utf-8" ?> <siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >     <siteMapNode url="Top" title="Top"  description="">      
   <siteMapNode  title="Admin"  description="" roles="admin">             <siteMapNode url="~/Admin/Admin.aspx" title="Admin"  description="" roles="Admin" />         </siteMapNode>      
   <siteMapNode  title="Users"  description="" roles="Users">    &nb ...