proposition: Lemma low_catB n2 n1 (p:BITS n1) (q:BITS n2) : low n2 (p ## q) = q.
proof: Proof. induction n2; first apply trivialBits. case/tupleP: q => x q. rewrite /catB catCons /= beheadCons. by rewrite IHn2. Qed.
imports: Coq(ZArith.ZArith) mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq fintype tuple div zmodp ssralg) ssrextra.nat ssrextra.tuple spec.spec