he following 'kernels' K:
</p><pre>
 KERNEL  K(i,j)
 ---------------------------------------------------------
  KL2    sum X .* Y
  KL1    sum min (X, Y)
  KCHI2  2 * sum (X .* Y) ./ (X + Y)
  KHELL  (X .* Y) .^ 0.5
</pre><p>
The constant are chosen so that D(i,j) = K(i,i) + K(j,j) - 2 K(i,j)
where D is the metric corresponding to the kenrel (if the arguments
are non-negative vectors). Each kernel can be interpreted as the
inner product inducing the corresponding metric in an embedding of
the real space in