docs: def create_milestone(self, project_id, title, deadline, party_id, notify):
          """
          Creates a single milestone. To create multiple milestones in a single
          call, see the "create (batch)" function. To make a company responsible
          for the milestone, prefix the company id with a "c".
          """
          path = '/projects/%u/milestones/create' % project_id
          req = ET.Element('request')
          req.append(
              self._create_milestone_elem(title, deadline, party_id, notify))
          return self._request(path, req)