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