'Retrieve necessary challenges to satisfy server.'

def _choose_challenges(self, domains):

    logger.info('Performing    the    following    challenges:')
    for dom in domains:
        path = gen_challenge_path(self.authzr[dom].body.challenges, self._get_chall_pref(dom), self.authzr[dom].body.combinations)
        dom_achalls = self._challenge_factory(dom, path)
        self.achalls.extend(dom_achalls)
