pre>
engine = enter_evidence(engine, evidence);
m = marginal_nodes(engine, nodes, t);
</pre>
Note: you must declare the observed nodes in the bnet before using
hmm_2TBN_inf_engine. 


<p>
Unfortunately, when all the hiddden nodes are discrete,
exact inference takes O(2^n) time, where n is the number of hidden
nodes per slice,
even if the model is sparse.
The basic reason for this is that two nodes become correlated, even if
there is no direct connection between them in the 2TBN,
by virtue of sharing common 