pub fn lam<A: Prop, B: Prop, X: Prop, C: Prop>( _ty_c: Ty<C, X>, ) -> Eq<App<Lam<Ty<A, X>, B>, C>, Subst<B, A, C>>
(c : x) => ((\(a : x) = b)(c) == b[a := c]).
(c : x) => ((\(a : x) = b)(c) == b[a := c])