constexpr decltype(auto) operator()(T1 && x1, T2 && x2, Ts && ...xs) const { return f_(g_(std::forward<T1>(x1)), g_(std::forward<T2>(x2)), std::forward<Ts>(xs)...); }
decltype(auto) operator()(const X& x) const { return g_(f_(x)); }
constexpr decltype(auto) operator()(Ts && ...xs) const { return f_( g_(), std::forward<Ts>(xs)...); }
int Ionosphere::g(real *param, real *funct, real *x) { return g_(param, funct, x); } /* g */