code: def _dens(self,R,z,phi=0.,t=0.):
          
          x,y,z= self._compute_xyz(R,phi,z,t)
          zc= numpy.sqrt(z**2.+self._c2)
          bzc2= (self._b+zc)**2.
          bigA= self._b*y**2.+(self._b+3.*zc)*bzc2
          bigC= y**2.+bzc2
          return self._c2/24./numpy.pi/self._a/bigC**2./zc**3.\
              *((x+self._a)*(3.*bigA*bigC+(2.*bigA+self._b*bigC)*(x+self._a)**2.)\
                    /(bigC+(x+self._a)**2.)**1.5\
               -(x-self._a)*(3.*bigA*bigC+(2.*bigA+self._b*bigC)*(x-self._a)**2.)\
                    /(bigC+(x-self._a)**2.)**1.5)
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:
             2016-11-04 - Written - Bovy (UofT/CCA)