code: def default_commit_veto(request, response):
      
      xtm = response.headers.get('x-tm')
      if xtm is not None:
          return xtm != 'commit'
      return response.status.startswith(('4', '5'))
docs: When used as a commit veto, the logic in this function will cause the
      transaction to be aborted if:
  
      - An ``X-Tm`` response header with the value ``abort`` (or any value
        other than ``commit``) exists.
  
      - The response status code starts with ``4`` or ``5``.
  
      Otherwise the transaction will be allowed to commit.