docs: def start_transaction(self):
          """Begin a transaction to group operations on the refpkg.
  
          All the operations until the next call to
          ``commit_transaction`` will be recorded as a single operation
          for rollback and rollforward, and recorded with a single line
          in the log.
          """
          if self.current_transaction:
              raise ValueError("There is already a transaction going")
          else:
              initial_state = copy.deepcopy(self.contents)
              self.current_transaction = {'rollback': initial_state,
                                          'log': '(Transaction left no log message)'}