code: def p_ext_doc_refs_1(self, p):
          
          try:
              if six.PY2:
                  doc_ref_id = p[2].decode(encoding='utf-8')
                  doc_uri = p[3].decode(encoding='utf-8')
                  ext_doc_chksum = p[4].decode(encoding='utf-8')
              else:
                  doc_ref_id = p[2]
                  doc_uri = p[3]
                  ext_doc_chksum = p[4]
  
              self.builder.add_ext_doc_refs(self.document, doc_ref_id, doc_uri,
                                            ext_doc_chksum)
          except SPDXValueError:
              self.error = True
              msg = ERROR_MESSAGES['EXT_DOC_REF_VALUE'].format(p.lineno(2))
              self.logger.log(msg)
docs: ext_doc_ref : EXT_DOC_REF DOC_REF_ID DOC_URI EXT_DOC_REF_CHKSUM