how to perform LogOut operation in silverlight

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Silverlight4

in silverlight xaml,i created one registration form for users(data stored in Sql server),after that i login application,i used Form Authentication.these all are okay.but
now i want to Log out from that application.

In asp.net we do like this
Session.Abandon();

but in silverlight,it doesn't support any Session.
i tried following all
WebContext.Current.Authentication.Logout(false);-->not working
or
HtmlPage.docoument.Submit();

please give me clear solution for logout in silverlight..