code: def parse_ext_doc_ref(self, ext_doc_ref_term):
          
          for _s, _p, o in self.graph.triples(
                  (ext_doc_ref_term,
                   self.spdx_namespace['externalDocumentId'],
                   None)):
              try:
                  self.builder.set_ext_doc_id(self.doc, six.text_type(o))
              except SPDXValueError:
                  self.value_error('EXT_DOC_REF_VALUE', 'External Document ID')
                  break
  
          for _s, _p, o in self.graph.triples(
                  (ext_doc_ref_term,
                   self.spdx_namespace['spdxDocument'],
                   None)):
              try:
                  self.builder.set_spdx_doc_uri(self.doc, six.text_type(o))
              except SPDXValueError:
                  self.value_error('EXT_DOC_REF_VALUE', 'SPDX Document URI')
                  break
  
          for _s, _p, checksum in self.graph.triples(
                  (ext_doc_ref_term, self.spdx_namespace['checksum'], None)):
              for _, _, value in self.graph.triples(
                      (checksum, self.spdx_namespace['checksumValue'], None)):
                  try:
                      self.builder.set_chksum(self.doc, six.text_type(value))
                  except SPDXValueError:
                      self.value_error('EXT_DOC_REF_VALUE', 'Checksum')
                      break
docs: Parses the External Document ID, SPDX Document URI and Checksum.