code: def set_spdx_doc_uri(self, doc, spdx_doc_uri):
          
          if validations.validate_doc_namespace(spdx_doc_uri):
              doc.ext_document_references[-1].spdx_document_uri = spdx_doc_uri
          else:
              raise SPDXValueError('Document::ExternalDocumentRef')
docs: Sets the `spdx_document_uri` attribute of the `ExternalDocumentRef`
          object.