import java.util.*;

public interface Combiner
{

   /**
       Apply a given row of a graph to a given column of a
         (possibly different graph) to get a candidate
         edge weight for the corresponding row and column
         of a third graph (which may be the same as one of the
         first two).
       @param row the list of edge weights of the row
       @param col
       @return the candidate weight
   */

   public Object apply(List row, List col);


   /**
       Determine the better value, given the choice of
       an old value and a new value.  The intended application
       is to return the better of an existing weight in a graph,
       or a newer canddiate value computed by <code>apply</code>
      @param current the old value
      @param newer the new value
      @return whichever of the two arguments is a better edge
       weight for the current application
   */
       
   public Object choose(Object current, Object newer);
}