proposition: Lemma jcPr_1 (A B : finType) (P : {fdist A * B}) a : P`1 a != 0 -> \sum_(b in B) \Pr_(fdistX P)[ [set b] | [set a] ] = 1.
proof: Proof. move=> Xa0; rewrite -[RHS](FDist.f1 (P `(| a ))); apply eq_bigr => b _. by rewrite jfdist_condE. Qed.
imports: mathcomp(all_ssreflect ssralg matrix) mathcomp(Rstruct) Reals ssrR realType_ext Reals_ext logb ssr_ext ssralg_ext bigop_ext fdist proba