proposition: Lemma sum_nat_cond_const pA n : \sum_(x | pA x) n = #|[set x | pA x]| * n.
proof: Proof. by rewrite sum_nat_const cardsE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)