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