1. A method of decoding an input semantic structure to generate an output semantic structure, the method comprising: providing a set of transfer mappings that cover at least portions of an input semantic structure that relates to an input word string of a first language, each transfer mapping having an input semantic side that describes at least one nodes of the input semantic structure and having an output semantic side that describes at least one node of the output semantic structure; using a processor to calculate a score for each of the set of transfer mappings which cover at least a select node of the input semantic structure using a statistical model, wherein calculating the score for each transfer mapping comprises combining scores of the highest scoring mappings for each child node of the select node not covered by the transfer mapping with the score of the transfer mapping; using the processor to select the highest scoring transfer mapping of the set of transfer mappings which cover the at least one select node; and using the processor to construct an output semantic structure that relates to an output word string of a second language using the selected highest scoring transfer mapping.