code: def submit_string(self):
          
          required = []
          optional = []
          for file_verifier in self.file_verifiers:
              if file_verifier.optional:
                  optional.append('[{0}]'.format(file_verifier.filename))
              else:
                  required.append(file_verifier.filename)
          return ' '.join(sorted(required) + sorted(optional))
docs: Return a string specifying the files to submit for this project.