descant
/ˈdeskænt; `dɛskænt/ n
(music 音) treble accompaniment (often improvised) which is sung or played to a melody 高音部伴奏或伴唱（常为即兴的）. descant, v / dɪˈsknt; dɪ`skænt/ [Ipr]
~ on/upon sth
1 (music 音) sing or play a descant on sth 为某乐曲作高音伴奏或伴唱.
2 (fml 文) talk for a long time about sth; comment on sth 长时间谈论某事; 评论某物
descant endlessly on the Government's failings 无尽无休地谈论政府的过失.