code: def addvPPfunc(self,solution):
          '''
          Adds the marginal marginal value function to an existing solution, so
          that the next solver can evaluate vPP and thus use cubic interpolation.
  
          Parameters
          ----------
          solution : ConsumerSolution
              The solution to this single period problem, which must include the
              consumption function.
  
          Returns
          -------
          solution : ConsumerSolution
              The same solution passed as input, but with the marginal marginal
              value function for this period added as the attribute vPPfunc.
          '''
          vPPfuncNow        = MargMargValueFunc(solution.cFunc,self.CRRA)
          solution.vPPfunc  = vPPfuncNow
          return solutiof addvPPfunc(self,solution):
          '''
          Adds the marginal marginal value function to an existing solution, so
          that the next solver can evaluate vPP and thus use cubic interpolation.
  
          Parameters
          ----------
          solution : ConsumerSolution
              The solution to this single period problem, which must include the
              consumption function.
  
          Returns
          -------
          solution : ConsumerSolution
              The same solution passed as input, but with the marginal marginal
              value function for this period added as the attribute vPPfunc.
          '''
          vPPfuncNow        = MargMargValueFunc(solution.cFunc,self.CRRA)
          solution.vPPfunc  = vPPfuncNow
          return solution
docs: Adds the marginal marginal value function to an existing solution, so
          that the next solver can evaluate vPP and thus use cubic interpolation.
  
          Parameters
          ----------
          solution : ConsumerSolution
              The solution to this single period problem, which must include the
              consumption function.
  
          Returns
          -------
          solution : ConsumerSolution
              The same solution passed as input, but with the marginal marginal
              value function for this period added as the attribute vPPfunc.