n on column jcol, and
   decide the supernode boundary.</pre><p>
<pre>   This routine does not use numeric values, but only use the RHS 
   row indices to start the dfs.</pre><p>
<pre>   A supernode representative is the last column of a supernode.
   The nonzeros in U[*,j] are segments that end at supernodal
   representatives. The routine returns a list of such supernodal 
   representatives in topological order of the dfs that generates them.
   The location of the first nonzero in each such supernodal s