ot tied, so each equivalence class for nodes 1..12 contains a
single node.
<p>
Above we assumed P(Y(i,t) | X(i,t)) is independent of t for t>1, but
it is conventional to assume this is true for all t.
So we would like to put N9, N21, N33, ... all in the same class, and
similarly for the other observed nodes.
We can do this by explicitely defining the equivalence classes, as
follows.
<p>
We define eclass1(i) to be the equivalence class that node i in slice
1 belongs to.
Similarly, we define eclass2(i) to be 