code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if self.alpha == 2.:
              return nu.log(R**2.+z**2.)/2. 
          else:
              return -(R**2.+z**2.)**(1.-self.alpha/2.)/(self.alpha-2.)
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:
             2010-07-10 - Started - Bovy (NYU)