proposition: Lemma polyseqC c : c%:P = nseq (c != 0) c :> seq R.
proof: Proof. by rewrite val_insubd /=; case: (c == 0). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)