code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          r2= R**2+z**2
          if r2 <= self.a2:
              return -1./self.a
          else:
              return -1./nu.sqrt(r2)
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:
             2018-08-04 - Written - Bovy (UofT)