We will try to guess the wxr version used
        to complete the wordpress xml namespace name.

def guess_wxr_version(self, tree):
        """
        We will try to guess the wxr version used
        to complete the wordpress xml namespace name.
        """
        for v in ('1.2', '1.1', '1.0'):
            try:
                tree.find('channel/{%s}wxr_version' % (WP_NS % v)).text
                return v
            except AttributeError:
                pass
        raise CommandError('Cannot resolve the wordpress namespace')