def formula(foyer_fiscal, period, parameters):
    """
        Renvoie True si le foyer est imposable, False sinon
        """
    iai = foyer_fiscal('iai', period)
    credits_impot = foyer_fiscal('credits_impot', period)
    cehr = foyer_fiscal('cehr', period)
    return iai - credits_impot + cehr > 0
