#[local] Instance Op_max_length : ZifyClasses.CstOp max_length :=
  { TCst := 16777211%Z ; TCstInj := eq_refl }.#[local] Ltac case_if :=
  lazymatch goal with
  | |- context C [if ?b then _ else _] => destruct b eqn:?
  | H : context C [if ?b then _ else _] |- _ => destruct b eqn:?
  end.

Lemma make_spec_valid_length (i : int) (c : char63) : to_Z i <= to_Z max_length -> to_list (make i c) = List.repeat (c land 255)%uint63 (to_nat i).