#[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 sub_get_spec (s : string) (off len i : int) : to_Z off + to_Z i < wB -> to_Z i < to_Z len -> get (sub s off len) i = get s (off + i).