Hint Rewrite @subst_lift1 (*@subst_lift1'*) @subst_lift1C  : subst.

Lemma subst_lift2':
forall {A1 A2 B} id v (f: A1 -> A2 -> B) a b,
subst id v (fun rho => f (a rho) (b rho)) = fun rho => f (subst id v a rho) (subst id v b rho).