 Complete the following phrase:  In 937, Dương Đình Nghệ was killed by his general Kiều Công Tiễn 矯公羨, who took over Jinghai. In 938, when another former general of Duongs, Ngô Quyền 吳權, rose at Ai Prefecture and subsequently attacked Jiao Prefecture, Kieu Cong Tien sought aid from Southern Han. Liu Yan wanted to use this opportunity to take over Jinghai again, so he commissioned his son Liu Hongcao as the military governor of Jinghai and changed his title to Prince of Jiao, having him command an army to head to Jiao Prefecture while Liu Yan himself commanded a follow-up army. By the time that Liu Hongcao was approaching Jiao Prefecture, however, Wu had already defeated and killed Kieu Cong Tien and occupied Jiao Prefecture. When Liu Hongcao prepared to attack, Wu set a trap for himsetting large wooden planks covered with iron into the tidal zone, such that when Liu Hongcao attacked at high tide, the planks were invisible, but when the tide fell, the Southern Han ships became stuck on the planks and unable to move. Ngo then counterattacked, killing more than half of the Southern Han soldiers, including Liu Hongcao. Liu Yan, hearing the news, cried bitterly and withdrew his own fleet. This became known as the 