YESTERDAY'S meeting was the closest thing to a formal court hearing that can be held in the Legco chamber. The Powers and Privileges Ordinance, which allows legislators to summon to testify before them anyone they wish, was invoked for the first time to investigate why Mr Tsui was dismissed. Security panel chairman Selina Chow Liang Shuk-yee opened the session by telling reporters they should seek legal advice on what they could write, because anything they reported might not be fully protected, as is usually the case in council hearings. ICAC Commissioner Bertrand de Speville swore, hand on Bible, that he would reveal ''the truth, the whole truth and nothing but the truth''. When legislators wanted a copy of Mr de Speville's speech on his five reasons for sacking Mr Tsui, they had to agree to have each of the copies numbered and signed. The procedures sought to ensure only one thing - that none of the copies would be leaked even though the speech had been read aloud by Mr de Speville word for word during the four-hour hearing. Public opinion once seemed to be on the side of Mr Tsui, the most senior officer to be dismissed under the sweeping Section 8 (2) of the ICAC Ordinance, which allows the explanation for his sacking to be kept secret. But what Mr de Speville said seemed to satisfy some legislators attending the hearing, at least for yesterday. United Democrat and deputy chairman of the security panel James To Kun-sun said: ''I find Mr de Speville a very helpful witness.'' Mr Tsui will testify today, but that probably won't be the end of the story. Mr To has already said the panel might need more hearings to summon people named by Mr de Speville, such as Henfrey Tin and Hung Wing-wah of the Hong Kong Boxing Association.