#[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_length_spec (s1 s2 : string) : length (cat s1 s2) = Uint63.min max_length (length s1 + length s2)%uint63.