'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)'

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

    return Traceback(filter(fn, self), self._excinfo)
