code: def _phiforce(self,R,z,phi=0.,t=0.):
          
          if self.isNonAxi:
              Rt2= R**2.*(1.-self._1m1overb2*nu.sin(phi)**2.)
              return R**2./(Rt2+(z/self._q)**2.+self._core2)\
                  *nu.sin(2.*phi)*self._1m1overb2/2.
          else:
              return 0
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuthal force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the azimuthal force
          HISTORY: