code: def create_external_document_ref_node(self, ext_document_references):
          
          ext_doc_ref_node = BNode()
          type_triple = (ext_doc_ref_node, RDF.type, self.spdx_namespace.ExternalDocumentRef)
          self.graph.add(type_triple)
  
          ext_doc_id = Literal(
              ext_document_references.external_document_id)
          ext_doc_id_triple = (
              ext_doc_ref_node, self.spdx_namespace.externalDocumentId, ext_doc_id)
          self.graph.add(ext_doc_id_triple)
  
          doc_uri = Literal(
              ext_document_references.spdx_document_uri)
          doc_uri_triple = (
              ext_doc_ref_node, self.spdx_namespace.spdxDocument, doc_uri)
          self.graph.add(doc_uri_triple)
  
          checksum_node = self.create_checksum_node(
              ext_document_references.check_sum)
          self.graph.add(
              (ext_doc_ref_node, self.spdx_namespace.checksum, checksum_node))
  
          return ext_doc_ref_node
docs: Add and return a creation info node to graph