def get_ref_annotation_at_time(self, tier, time):
        """Give the ref annotations at the given time of the form
        ``[(start, end, value, refvalue)]``

        :param str tier: Name of the tier.
        :param int time: Time of the annotation of the parent.
        :returns: List of annotations at that time.
        :raises KeyError: If the tier is non existent.
        """
        bucket = []
        for aid, (ref, value, _, _) in self.tiers[tier][1].items():
            begin, end, rvalue, _ = self.tiers[self.annotations[ref]][0][ref]
            begin = self.timeslots[begin]
            end = self.timeslots[end]
            if begin <= time and end >= time:
                bucket.append((begin, end, value, rvalue))
        return bucket
Give the ref annotations at the given time of the form
        ``[(start, end, value, refvalue)]``

        :param str tier: Name of the tier.
        :param int time: Time of the annotation of the parent.
        :returns: List of annotations at that time.
        :raises KeyError: If the tier is non existent.