void measure_ (const RenyiModPossibleWalk &walk)
     {
         estimate.add_value(std::abs(walk.get_phibeta1().psi().ratio(walk.get_phialpha1().psi())
                                     * walk.get_phibeta2().psi().ratio(walk.get_phialpha2().psi())));
     }
 virtual void measure_ (const RenyiModPossibleWalk &walk) override
 {
     using std::abs;
     estimate.add_value(abs(walk.get_phibeta1().psi().ratio(walk.get_phialpha1().psi())
                            * walk.get_phibeta2().psi().ratio(walk.get_phialpha2().psi())));
 }