(* HIDE *)
Fixpoint aeq_f  (n: nat) (L : atoms) (t1 : n_exp) (t2 : n_exp) : bool :=
match n with
| 0 => false
| S m =>
match t1 , t2 with
| n_var x , n_var y => if (x == y) then true else false
| n_abs x1 t1, n_abs x2 t2 =>
if (x1 == x2) then aeq_f m L t1 t2
else let (y,_) := atom_fresh L in
aeq_f m ({{y}} \u L) (swap y x1 t1) (swap y x2 t2)
| n_app t1 t2 , n_app t1' t2' =>
if aeq_f m L t1 t1' then aeq_f m L t2 t2' else false
| _ , _ => false
end
end.
Definition is_aeq t1 t2 := aeq_f (size t1) (fv_nom t1 \u fv_nom t2).
(************)

Lemma subst_equivariance : forall u x y z t,
swap x y (subst u z t) = subst (swap x y u) (swap_var x y z) (swap x y t).