pub fn fun_ext<F: Prop, G: Prop, X: Prop, Y: Prop, A: Prop>( tauto_eq_fg: Tauto<Eq<F, G>>) -> FunExtTy<F, G, X, Y, A>
(f == g)^true => fun_ext_ty(f, g).
(f == g)^true => fun_ext_ty(f, g)