public static String Delete ()
     {
         String matrix = matches[0].Groups[2].ToString();
         if (Calculatrice.listMatrix.ContainsKey(matrix))
         {
             Matrix m = Calculatrice.listMatrix[matrix];

             int index = Calculatrice.mainWindow.ListBox_display.Items.IndexOf(m);

             Calculatrice.mainWindow.ListBox_display.Items.RemoveAt(index);

             Calculatrice.listMatrix.Remove(matrix);

             return "Done!n";
         }
         throw new Exception("The matrix does not exist.n");
     }