docs: def build(self, builder):
          """Build XML by appending to builder"""
          params = dict(StudyEventOID=self.oid, Mandatory=bool_to_yes_no(self.mandatory))
          if self._order_number is not None:
              params["OrderNumber"] = self._order_number
          builder.start("StudyEventRef", params)
          builder.end("StudyEventRef")