rginals on a specified set of clusters. For example, 
in the water network, we might use the following clusters:
<pre>
engine = bk_inf_engine(bnet, { [1 2], [3 4 5 6], [7 8] });
</pre>
This engine can now be used just like the jtree engine.
Two special cases of the BK algorithm are supported: 'ff' (fully
factored) means each node has its own cluster, and 'exact' means there
is 1 cluster that contains the whole slice. These can be created as
follows:
<pre>
engine = bk_inf_engine(bnet, 'ff');
engine = bk_inf_