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