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.