def _missing_co2_params(params, *args, _not=False):
    """
    Checks if all co2_params are not defined.

    :param params:
        CO2 emission model parameters (a2, b2, a, b, c, l, l2, t, trg).
    :type params: dict | lmfit.Parameters

    :param _not:
        If True the function checks if not all co2_params are defined.
    :type _not: bool

    :return:
        If is missing some parameter.
    :rtype: bool
    """
    s = {'a', 'b', 'c', 'a2', 'b2', 'l', 'l2', 't0', 'dt', 'trg'}
    if _not:
        return set(params).issuperset(s)
    return not set(params).issuperset(s)
