def default_commit_veto(request, response):
    """
    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.
    """
    xtm = response.headers.get('x-tm')
    if xtm is not None:
        return xtm != 'commit'
    return response.status.startswith(('4', '5'))