def _evaluate_results(results):
    """ Evaluate resulting assertions.

    Evaluation is OK if all assertions are true, otherwise it is KO

    Parameters
    ----------
    results: list(bool)
        List of all assertion execution results.

    Returns
    -------
    str
        'OK' if all assertions are true, 'KO' otherwise
    """
    if len(results) and results.count(True) == len(results):  # all True
        return 'OK'
    else:
        return 'KO'