proposition: Lemma mask_cat m1 m2 s1 s2 : size m1 = size s1 -> mask (m1 ++ m2) (s1 ++ s2) = mask m1 s1 ++ mask m2 s2.
proof: Proof. by move: m1 s1; apply: seq_ind2 => // -[] m1 x1 s1 /= _ ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)