function b = back(engine, bfuture, f, t)if f.t ~= t  error('mixed up time stamps')endb.t = t;b.obslik = f.obslik;bb_future = bfuture.beta .* bfuture.obslik;if engine.maximize  B = repmat(bb_future(:)', length(bfuture.beta), 1);  b.beta = normalise(max(engine.transprob .* B, [], 2));else  b.beta = normalise((engine.transprob * bb_future));endb.gamma = normalise(f.alpha .* b.beta);if t > 1  bb_t = b.beta .* b.obslik;  b.xi = normalise((engine.transprob .* (f.past_alpha * bb_t'))); % t-1,tend