intuition.
exists (ret (a, b)).
intuition.
unfold marginal_l.
dist_compute.
unfold marginal_r.
dist_compute.
simp_in_support.
trivial.
Qed.

Theorem comp_spec_rnd :
forall n x y,
comp_spec (fun a b : Vector.t bool n => a = x <-> b = y)
({ 0 , 1 }^n) ({ 0 , 1 }^n).