proposition: Lemma sum1dep_card pA : \sum_(x | pA x) 1 = #|[set x | pA x]|.
proof: Proof. by rewrite sum1_card cardsE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)