proposition: Fact poly_mul_comm p q : p * q = q * p.
proof: Proof. apply/polyP=> i; rewrite coefM coefMr. by apply: eq_bigr => j _; rewrite mulrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)