#[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 string_eq_ext (s1 s2 : string) : (length s1 = length s2 /\ forall i, to_Z i < to_Z (length s1) -> get s1 i = get s2 i) -> s1 = s2.