code: def _phiforce(self,R,phi=0.,t=0.):
          
          if R < self._rb:
              return self._mphio*math.sin(self._m*phi-self._mphib)\
                  *self._rbp*(2.*self._r1p-self._rbp/R**self._p)
          else:
              return self._mphio*R**self._p*math.sin(self._m*phi-self._mphib)
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuthal force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             phi - azimuth
             t - time
          OUTPUT:
             the azimuthal force
          HISTORY:
             2011-10-19 - Written - Bovy (IAS)