One common limitation of the above works is they only allow the one-to-one mapping between each non-terminal frontier node, and thus they suffer from the issue of rule coverage.
due to the data sparseness issue and model coverage issue, previous tree-to-tree (Zhang et al., 2008;Liu et al., 2009) decoder has to rely solely on the span information or source side information to combine the target syntactic structures, without checking the compatibility of the merging nodes, in order not to fail many translation paths.