@property<EOL><INDENT>def s2(self):<DEDENT>
return self._s2<EOL>
Property of second legendre polynomial factor s2.

        s2 in following equation:

        .. math::

            S(\\varphi) = \\frac{S_0}{4} \\left( 1 + s_2 P_2(x) \\right)

        :getter:    Returns the s2 parameter which is stored in attribute ``self._s2``.
        :setter:    * sets s2 which is addressed as ``self._S0`` to the new value
                    * updates the parameter dictionary ``self.param['s2']`` and
                    * calls method :func:`_compute_fixed`
        :type:      float