code: def create_doc(self):
          
          doc_node = URIRef('http://www.spdx.org/tools#SPDXRef-DOCUMENT')
          # Doc type
          self.graph.add((doc_node, RDF.type, self.spdx_namespace.SpdxDocument))
          # Version
          vers_literal = Literal(str(self.document.version))
          self.graph.add((doc_node, self.spdx_namespace.specVersion, vers_literal))
          # Data license
          data_lics = URIRef(self.document.data_license.url)
          self.graph.add((doc_node, self.spdx_namespace.dataLicense, data_lics))
          doc_name = URIRef(self.document.name)
          self.graph.add((doc_node, self.spdx_namespace.name, doc_name))
          return doc_node
docs: Add and return the root document node to graph.