In the pair model, the position pair of (i, j) is used to derive features.
to descriminate label sequences in the sequence model, the position pairs of (i, k), k ∈ {k|i < k ≤ j ∨ j ≤ k < i} and (k, j), k ∈ {k|i ≤ k < j ∨ j < k ≤ i} are used to derive features.