docs: def build(self, builder):
          """Build XML by appending to builder"""
          params = dict(StudyOID=self.study_oid,
                        MetaDataVersionOID=self.metadata_version_oid,
                        EffectiveDate=dt_to_iso8601(self.effective_date))
          builder.start("MetaDataVersionRef", params)
          builder.end("MetaDataVersionRef")