proposition: Lemma collb_sym : symmetric collb.
proof: Proof. move => x y. by apply/collP/collP; move => H w; rewrite H. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph finfun finset generic_quotient) RegLang(misc languages dfa)