For this feature and the next one, we extend the summation in (1) to allow 0 ≤ i < j ≤ n + 1, taking t 0 = r 0 = BOS ("beginning of sequence") and t n+1 = r n+1 = EOS ("end of sequence").
a bigram feature such as A.DET.EOS would fire on DET when it falls at the end of the sequence.