It is easy to see that 1 − U AM A satisfies all the requirements of a metric, i.e., it is non-negative, equals zero if and only if ∼ r =∼ p , symmetric, and obeys the triangle inequality, since if the triangle inequality holds for 1 − U set agreement , it must hold for a sum of 1 − U set agreement values.
unlike sequence alignments, there are no ordering constraints in word alignments, and the alignments are many-to-many as opposed to one-to-one.