code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
          xy= np.dot(self.rot(t),np.array([x,y]))
          x,y= xy[0],xy[1]
          return self._evaluate_xyz(x,y,z)
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)