code: public double dot(int k, KernelPoints X, int j)
      {
          return points.get(k).dot(X.points.get(j));
      }
docs: Computes the dot product between the {@code k}'th KernelPoint and the
  {@code j}'th KernelPoint in the given set of points.
  @param k the index of the KernelPoint in this set to contribute to the
  dot product
  @param X the other set of KernelPoints
  @param j the index of the KernelPoint in the given set to contribute to
  the dot product
  @return the dot product between the {@code k}'th KernelPoint and the
  {@code j}'th KernelPoint in the given set