Definition sum_dist {A} (l : list (A * Q)) : Q :=
List.fold_right Qplus 0 (List.map snd l).
Record dist (A : Type) : Type :=
Dist
{ dist_pmf : list (A * Q);
dist_law : Qeq (sum_dist dist_pmf) 1
}.
Arguments Dist {A} _.
Arguments dist_pmf {A} _.
Definition mreturn_dist {A : Type} (x : A) : dist A :=
{| dist_pmf := [(x, 1 / 1)];
dist_law := eq_refl;
|}.

Lemma refold_sum_dist:
forall {A} (a : A) (q : Q) (l : list (A * Q)),
sum_dist ((a, q) :: l) = q + sum_dist l.