code: def p_file_artifact(self, f_term, predicate):
          
          for _, _, project in self.graph.triples((f_term, predicate, None)):
              if (project, RDF.type, self.doap_namespace['Project']):
                  self.p_file_project(project)
              else:
                  self.error = True
                  msg = 'File must be artifact of doap:Project'
                  self.logger.log(msg)
docs: Handles file artifactOf.
          Note: does not handle artifact of project URI.