Former legal scholar Benny Tai Yiu-ting had elaborated his plan to use a "highly lethal constitutional weapon" to force then chief executive Carrie Lam Cheng Yuet-ngor to dismiss the Legislative Council and step down during a meal gathering in January 2020, when the idea of organizing a primary Legco election among the pro-democracy camp was first raised, ex-lawmaker Au Nok-hin told a court yesterday.