docs: def commit_transaction(self, log=None):
          """Commit a transaction, with *log* as the log entry."""
          self.current_transaction['rollback'].pop('log')
          self.current_transaction['rollback'].pop('rollforward')
          self.contents['log'].insert(
              0, log and log or self.current_transaction['log'])
          self.contents['rollback'] = self.current_transaction['rollback']
          self.contents['rollforward'] = None  # We can't roll forward anymore
          self.current_transaction = None
          self._sync_to_disk()