Inspired by feature engineering for Conditional Random Fields (CRFs) (Lafferty et al., 2001), we design two kinds of features: unigram (state) features defined on each letter mapping, and bigram (transition) features defined on each pair of adjacent letter mappings.
given a triplet, a, x, and y, a global feature function f k (a, x, y) ∈ F sums up the boolean values (0 or 1) of the corresponding local feature ( 3)f k (a, x, y) counts the number of times the local feature is fired in the alignment a.