proposition: Lemma hornerXsubC a x : ('X - a%:P).[x] = x - a.
proof: Proof. by rewrite hornerD hornerN hornerC hornerX. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)