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 order for one-to-many alignment methods to be effective, each f j must contain enough information to allow for effective alignment with its corresponding elements in e I 1 .