'Whether or not the header has been modified; this is a property so that
it can also check each card for modifications--cards may have been
modified directly without the header containing it otherwise knowing.'

@property
def _modified(self):

    modified_cards = any((c._modified for c in self._cards))
    if modified_cards:
        self.__dict__['_modified'] = True
    return self.__dict__['_modified']
