code: def diffse(self, x1, x2):
           
          f1, f1se = self(x1)
          f2, f2se = self(x2)
          if self.paired:
              fx1 = np.array(self.cache[tuple(x1)])
              fx2 = np.array(self.cache[tuple(x2)])
              diffse = np.std(fx1-fx2, ddof=1)/self.N**.5 
              return diffse
          else:
              return (f1se**2 + f2se**2)**.5
docs: Standard error of the difference between the function values at x1 and x2