code: def k_xx(self,t,tprime,s,sprime,l):
          
          return 0.5 * (l**2) * ( self.g((t-sprime)/l) + self.g((tprime-s)/l) - self.g((t - tprime)/l) - self.g((s-sprime)/l))
docs: Covariance between observed values.
  
          s and t are one domain of the integral (i.e. the integral between s and t)
          sprime and tprime are another domain of the integral (i.e. the integral between sprime and tprime)
  
          We're interested in how correlated these two integrals are.
  
          Note: We've not multiplied by the variance, this is done in K.