Hint Resolve subst_exp_close_exp_wrt_exp : lngen.
(* begin hide *)

Lemma subst_exp_degree_exp_wrt_exp_mutual :
(forall e1 e2 x1 n1,
degree_exp_wrt_exp n1 e1 ->
degree_exp_wrt_exp n1 e2 ->
degree_exp_wrt_exp n1 (subst_exp e2 x1 e1)).