code: def set_doc_namespace(self, doc, namespace):
          
          if not self.doc_namespace_set:
              self.doc_namespace_set = True
              if validations.validate_doc_namespace(namespace):
                  doc.namespace = namespace
                  return True
              else:
                  raise SPDXValueError('Document::Namespace')
          else:
              raise CardinalityError('Document::Comment')
docs: Sets the document namespace.
          Raise SPDXValueError if malformed value, CardinalityError
          if already defined.