wolffd@0: function b = backT(engine, f, t)
wolffd@0: 
wolffd@0: b.t = t;
wolffd@0: b.obslik = f.obslik;
wolffd@0: Q = length(f.alpha);
wolffd@0: b.beta = ones(Q,1);
wolffd@0: b.gamma = f.alpha;
wolffd@0: if t > 1
wolffd@0:   bb_t = b.obslik;
wolffd@0:   b.xi = normalise((engine.transprob .* (f.past_alpha * bb_t'))); % T-1,T
wolffd@0: end