(** If [Cat] matches a string, the string can be divided into two strings which match the two [RegExp] respectively. *)

(* Specifications: *)
Lemma divide_Cat : forall s r' r'', (r' ++ r'') ~== s ->
{s':string & {s'':string | s = (s' ++ s'')%string /\ r' ~== s' /\ r'' ~== s'' }}.