'Returns activity\'s index within a feed or raises ValueError if not present
:param key: the key at which the feed is stored
:param activity_id: the activity\'s id to search'

def index_of(self, key, activity_or_id):

    activity_id = self.activities_to_ids([activity_or_id])[0]
    return self.get_index_of(key, activity_id)
