intuition.
eapply refl.
eapply evalDist_assoc_eq.
Qed.

Theorem evalDist_commute:
forall rel  (A B : Set)(c1 : Comp A)(c2 : Comp B)(C : Set)(c3 : A -> B -> Comp C) x,
RatRel rel ->
rel (evalDist (a <-$ c1; b <-$ c2; (c3 a b)) x) (evalDist (b <-$ c2; a <-$ c1; (c3 a b)) x).