proposition: Lemma nth_mul_tail u n p q k : nth (mul_tail u n p q) k = if (size p + size q).-1 - n.+1 <= k then C.zero else mul_coeff u p q (n.+1 + k).
proof: Proof. by rewrite /nth /mul_tail [in LHS]nth_mkseq_dflt. Qed.
imports: Coq(ZArith Reals) Coquelicot(Coquelicot) mathcomp.ssreflect(ssreflect ssrfun ssrbool eqtype ssrnat seq fintype bigop) Flocq(Core) Stdlib MathComp Interval Xreal Basic_rec