code: def set_chksum(self, doc, chk_sum):
          
          if chk_sum:
              doc.ext_document_references[-1].check_sum = checksum.Algorithm(
                  'SHA1', chk_sum)
          else:
              raise SPDXValueError('ExternalDocumentRef::Checksum')
docs: Sets the external document reference's check sum, if not already set.
          chk_sum - The checksum value in the form of a string.