code: def _Rphideriv(self,R,phi=0.,t=0.):
          
          return self._Pot.Rphideriv(R,0.,phi=phi,t=t,use_physical=False)
docs: NAME:
             _Rphideriv
          PURPOSE:
             evaluate the mixed radial-azimuthal derivative
          INPUT:
             R
             phi
             t
          OUTPUT:
             d2phi/dRdphi
          HISTORY:
             2016-06-02 - Written - Bovy (UofT)