code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if not self.HernquistSelf == None:
              return self.HernquistSelf._evaluate(R,z,phi=phi,t=t)
          elif not self.JaffeSelf == None:
              return self.JaffeSelf._evaluate(R,z,phi=phi,t=t)
          elif not self.NFWSelf == None:
              return self.NFWSelf._evaluate(R,z,phi=phi,t=t)
          else:
              return TwoPowerSphericalPotential._evaluate(self,R,z,
                                                          phi=phi,t=t,
                                                          _forceFloatEval=True)
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2010-07-09 - Started - Bovy (NYU)