proposition: Lemma congr_catr u v1 v2 : r v1 v2 -> r (u ++ v1) (u ++ v2).
proof: Proof. Proof using Hcongr. by rewrite -[u ++ v1]cats0 -[u ++ v2]cats0 -!catA; exact: Hcongr. Qed.
imports: HB(structures) mathcomp(all_ssreflect) Recdef permcomp permuted multinomial vectNK