code: def _denom(self, R, z):
          
          return (R**2. + (self._a + nu.fabs(z))**2.)
docs: NAME:
             _denom
          PURPOSE:
             evaluate R^2 + (a + |z|)^2 which is used in the denominator
             of most equations
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
          OUTPUT:
             R^2 + (a + |z|)^2
          HISTORY:
             2016-05-09 - Written - Aladdin