This is done by decrementing the score of a sentence that belongs to an SC for which there is a higher or equal scoring sentence (if there are several highest-scoring sentences, we retain one sentence as a reference sentence-i.e., its score is not decremented).
given a sentence s k in cluster SC l which contains a sentence with a higher or equal score, the contribution of SC l to Score(s k ) (= Pr(SC l ) × Pr(s k |SC l )) is subtracted from Score(s k ).