code: def dKd_dLen(self, X, dimension, lengthscale, X2=None):
  		
  		r = self._scaled_dist(X, X2)
  		return self.dKdLen_of_r(r, dimension, lengthscale)
docs: 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)