proposition: Lemma map_comm_coef p x : comm_coef p x -> comm_coef p^f (f x).
proof: Proof. by move=> cpx i; rewrite coef_map -!rmorphM ?cpx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)