proposition: Lemma perm_catACA s1 s2 s3 s4 : perm_eql ((s1 ++ s2) ++ (s3 ++ s4)) ((s1 ++ s3) ++ (s2 ++ s4)).
proof: Proof. by apply/permPl; rewrite perm_catAC !catA perm_catAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)