code: def add_file_dependencies_helper(self, doc_file):
          
          subj_triples = list(self.graph.triples((None, self.spdx_namespace.fileName, Literal(doc_file.name))))
          if len(subj_triples) != 1:
              raise InvalidDocumentError('Could not find dependency subject {0}'.format(doc_file.name))
          subject_node = subj_triples[0][0]
          for dependency in doc_file.dependencies:
              dep_triples = list(self.graph.triples((None, self.spdx_namespace.fileName, Literal(dependency))))
              if len(dep_triples) == 1:
                  dep_node = dep_triples[0][0]
                  dep_triple = (subject_node, self.spdx_namespace.fileDependency, dep_node)
                  self.graph.add(dep_triple)
              else:
                  print('Warning could not resolve file dependency {0} -> {1}'.format(doc_file.name, dependency))
docs: Handle dependencies for a single file.
          - doc_file - instance of spdx.file.File.