right; congruence.
Qed.
Definition var := Ident.t.
Definition env := var -> nat.
Definition env_get (rho: env) : var -> val := fun x => Some (rho x).
Definition env_set (x: var) (v: val) (rho: env) : env :=
match v with
| None (*empty_val*) => rho
| Some v' => fun y => if Ident.eq_dec x y then v' else rho y
end.

Lemma gss_env (x: var) (v: val) (s: env) : v<>empty_val -> env_get (env_set x v s) x = v.
unfold env_get, env_set; intros.