11. The method of claim 10 wherein the tree based constraint comprises membership in one of a plurality of specified classes of tree-based structures.
10. A method for computer-assisted translation from a source language to a target language, the method comprising: accepting a plurality of rules, each rule forming an association between a representation of a sequence of source language tokens with a corresponding tree-based structure in the target language, wherein the tree-based structure for each rule satisfies a tree-based constraint and includes at least one target language token associated with one or more source language tokens in the sequence of source language tokens for the rule; and decoding, using a computer, an input sequence of source tokens according to the plurality of rules to generate a representation of one or more output sequences of target language tokens, including for each of at least some sub-sequences of the input sequence of source tokens, determining a tree-based structure associated with the sub-sequence according to a match to one of the plurality of rules, and for each of at least some sub-sequences of the input sequence of source tokens, selecting for combination a subset of determined tree-based structures, each associated with a sub-sequence, such that a tree-based structure formed as a combination of the subset of partial trees satisfies the tree-based constraint, and determining the tree-based structure as the combination of the selected subset of tree-based structures.