proposition: Lemma is_code_rconsK c i : rcons c i \is a code -> c \is a code.
proof: Proof. move/is_codeP; rewrite size_rcons => Hcode. apply/is_codeP => j Hj; have:= Hj; rewrite -ltnS => /ltnW/Hcode. by rewrite nth_rcons Hj. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr