internal static string CleanFileName( string s )
                {
                        System.Text.StringBuilder z= new System.Text.StringBuilder();
                        foreach (char c in s)
                        {
                                if (System.Char.IsLetterOrDigit(c))
                                {
                                        z.Append(System.Char.ToLower(c));
                                }
                                else
                                {
                                        //z.Append('%');
                                        z.AppendFormat("%{0:X2}", (int)c);
                                }
                        }
                        return z.ToString();
                }


#if DEBUG
        [TestFixture] public class PersonChatModelTests
        {
                [Test] public void TestCleanFileName_ToLower()
                {
                        NUnit.Framework.Assert.AreEqual(
                                "message2earth",
                                PersonChatModel.CleanFileName("Message2Earth")
                                );
                }

                [Test] public void TestCleanFileName_HexEscapes()
                {
                        NUnit.Framework.Assert.AreEqual(
                                "earth%20people%21joe%40milky%2Dway%2Egal",
                                PersonChatModel.CleanFileName("earth people!joe@milky-way.GAL")
                                );
                }
        }
#endif