In order to achieve this, we symmetrize Viterbi alignments for source-target and target-source directions by intersection only.
given two Viterbi alignments A 1 = {(a j , j)| a j > 0} and A 2 = {(i, b i )| b i > 0}, the alignments in the intersection are defined as A = A 1 ∩ A 2 .