proposition: Lemma prodrXr x I r P (F : I -> nat) : \prod_(i <- r | P i) x ^+ F i = x ^+ (\sum_(i <- r | P i) F i).
proof: Proof. by rewrite (big_morph _ (exprD _) (erefl _)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)