proposition: Lemma sum_ffunE x : (\sum_(i <- r | P i) F i) x = \sum_(i <- r | P i) F i x.
proof: Proof. by elim/big_rec2: _ => // [|i _ y _ <-]; rewrite !ffunE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)