In the formalism presented above, this means that each e i must be included in at most one span, and for each span u = v. Traditionally, these models are run in both directions and combined using heuristics to create many-to-many alignments (Koehn et al., 2003).
in addition, we heuristically prune values for which the conditional probabilities P (e|f ) or P (f |e) are less than some fixed value, which we set to 0.1 for the reported experiments.