code: def _dens(self,R,z,phi=0.,t=0.):
          
          #Cylindrical distance
          Rdist = _cylR(R,phi,self._orb.R(t),self._orb.phi(t))
          # Difference vector
          (xd,yd,zd) = _cyldiff(self._orb.R(t), self._orb.phi(t), self._orb.z(t),
              R, phi, z)
          # Return the density
          return evaluateDensities(self._pot, Rdist, zd, use_physical=False)
docs: NAME:
             _dens
          PURPOSE:
             evaluate the density for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the density
          HISTORY:
             2010-08-08 - Written - Bovy (NYU)