docs: def begin(self):
          """ Begin a transaction """
  
          if self.journal != None:
              raise Exception('Storage is already active, nested begin not supported')
  
          # under normal operation journal is deleted at end of transaction
          # if it does exist we need to roll back
          if os.path.isfile(self.j_file):  self.rollback()
  
          self.journal = open(self.j_file, 'w')