def filter(self, fn=lambda x: not x.ishidden()):
return Traceback(filter(fn, self), self._excinfo)<EOL>
return a Traceback instance with certain items removed

            fn is a function that gets a single argument, a TracebackEntry
            instance, and should return True when the item should be added
            to the Traceback, False when not

            by default this removes all the TracebackEntries which are hidden
            (see ishidden() above)