simpl; auto.
Qed.
Fixpoint iter {A: Type} (f: A -> A) (n: nat) (x: A) :=
match n with
| O => x
| S n1 => f (iter f n1 x)
end.
(* A bit of ssreflect *)
Coercion b2Prop (x : bool) := x = true.

Lemma minus0_le m n: m <= n -> m - n = 0.
Fixpoint eq_nat (m n: nat) :=
match m, n with (S m), (S n) => eq_nat m n | 0, 0 => true | _, _ => false end.