 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 segment
   (supernodal entry location) is also returned.</pre><p>
<pre> Local parameters
 ================
   nseg: no of