#[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_length_spec (s : string) (off len : int) : to_Z off <= to_Z (length s) -> to_Z len <= to_Z (length s) - to_Z off -> length (sub s off len) = len.