proposition: Lemma root_XsubC a x : root ('X - a%:P) x = (x == a).
proof: Proof. by rewrite rootE hornerXsubC subr_eq0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)