Probably you may find convenient reading two publications of mine:1) Alesii, Giuseppe. Esercizi e complementi di finanza aziendale Roma Aracne, 2008, find summary at
http://bit.ly/1awdE0FAs you may notice, in the appendix there are the GAUSS codes of (Boyle et al., 1989), (Kamrad and Ritchken, 1991), (Ekvall, 1996) and (Gamba and Trigeorgis, 2005).The last two models do use a decomposition approach. Although I am not sure this is what you are actually looking for.Within the text, a plain expanation of the decomposition approach by the last two models is reported with worked out examples.2) Alesii, Giuseppe, Going Parallel Over the Rainbow (December 8, 2013). Available at SSRN, find a copy of the paper at
http://bit.ly/1hILEMhThis paper reports a parallel version --pseudo codes only-- of the previous multivariate lattice models.I hope it helps.To be more supportive, it would be good to know the Hull and White reference as you may download it from Google Scholar, else poring on Hull "The Textbook" one may miss a model for another.