#[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 cat_get_spec_r (s1 s2 : string) (i : int) : to_Z (length s1) <= to_Z i -> to_Z i < to_Z max_length -> get (cat s1 s2) i = get s2 (i - length s1).