code: protected void aggregate(ClusKernel other, long timeDifference, double negLambda) {
          makeOlder(timeDifference, negLambda);
          add(other);
      }
docs: Make this cluster older bei weighting it and add to this cluster the
  given cluster. If we want to add somethin to the cluster, but don't
  want to weight it, we should use the function <code>add(Cluster)</code>.
  @param other The other cluster to be added to this one.
  @param timeDifference The time elapsed between the last update of the
  <code>Entry</code> to which this cluster belongs and the update that
  caused the call to this function.
  @param negLambda A parameter needed to weight the cluster.
  @see #add(tree.Kernel)