@Cache_this(limit=<NUM_LIT:3>, ignore_args=())<EOL><INDENT>def dKd_dLen(self, X, dimension, lengthscale, X2=None):<DEDENT>
r = self._scaled_dist(X, X2)<EOL>return self.dKdLen_of_r(r, dimension, lengthscale)<EOL>
Derivate of Kernel function wrt lengthscale applied on inputs X and X2.
In the stationary case there is an inner function depending on the
distances from X to X2, called r.

dKd_dLen(X, X2) = dKdLen_of_r((X-X2)**2)