code: def gradient(self):
          
          grad = {}
          for i, f in enumerate(self._covariances):
              for varname, g in f.gradient().items():
                  grad[f"{self._name}[{i}].{varname}"] = g
          return grad
docs: Sum of covariance function derivatives.
  
          Returns
          -------
          dict
              ∂K₀ + ∂K₁ + ⋯