(** * Arities  - [arities e] returns the arities (i.e., the number of normal and safe variables) of the expression [e] if it is well formed, or an error including the arities of the subterms. *)
Inductive Arities : Set :=
| error_rec : Arities -> Arities -> Arities -> Arities
| error_comp : Arities -> list Arities -> list Arities -> Arities
| error_proj : nat -> nat -> nat -> Arities
| ok_arities : nat -> nat -> Arities.
Definition aeq (x y:Arities) : bool :=
match x, y with
| ok_arities xn xs, ok_arities yn ys => beq_nat xn yn && beq_nat xs ys
| _, _ => false
end.

Lemma aeq_eq_conv xn yn xs ys :
aeq (ok_arities xn xs) (ok_arities yn ys) = false -> xn <> yn \/ xs <> ys.