def test_traceback_filter(self):

       traceback = self.excinfo.traceback

->     ntraceback = traceback.filter()

       assert len(ntraceback) == len(traceback) - 1

   def filter(self, fn=lambda x: not x.ishidden()):

       """ return a Traceback instance with certain items removed

               fn is a function that gets a single argument, a TracebackItem

               instance, and should return True when the item should be added

               to the Traceback, False when not

               by default this removes all the TracebackItems which are hidden

               (see ishidden() above)

           """

->     return Traceback(filter(fn, self))