proposition: Lemma poly_def n E : \poly_(i < n) E i = \sum_(i < n) E i *: 'X^i.
proof: Proof. by apply/polyP => i; rewrite coef_sumMXn coef_poly big_ord1_eq. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)