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")
Build XML by appending to builder