'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)'

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

    return Traceback(filter(fn, self))
