1. 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 of at least some of the rules represents one or more asymmetrical relations within a plurality of target tokens associated with the tree-based structure and provides an association of the plurality of target tokens with the sequence of source language tokens of the rule; and decoding, sing 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.
8. The method of claim 1 wherein each of the plurality of tree-based structures is associated with a target language model score that depends on the tree-based structure and target language tokens associated with the tree-based structure.