def add_milestone(self,
                      milestone,
                      codelistoid="MILESTONES"):
        """
        Add a milestone
        :param codelistoid: specify the CodeListOID (defaults to MILESTONES)
        :param str milestone: Milestone to add
        """
        if milestone not in self.milestones.get(codelistoid, []):
            self._milestones.setdefault(codelistoid, []).append(milestone)