intuition.
eapply (eqRat_ratAdd_same_r (evalDist c a)).
symmetry.
rewrite ratAdd_comm.
rewrite ratSubtract_ratAdd_inverse_2.
rewrite <- (evalDist_lossless H).
rewrite (sumList_filter_partition (eqb a)).
rewrite ratAdd_comm.
eapply ratAdd_eqRat_compat; intuition.
destruct (eq_Rat_dec (evalDist c a) 0).
rewrite e.
eapply sumList_0; intuition.
apply filter_In in H0.
intuition.
rewrite eqb_leibniz in H2; subst.
trivial.
rewrite (@sumList_exactly_one _ a); intuition.
eapply filter_NoDup.
eapply getSupport_NoDup.
eapply filter_In; intuition.
eapply getSupport_In_evalDist; intuition.
eapply eqb_refl.
apply filter_In in H0; intuition.
rewrite eqb_leibniz in H3; subst.
intuition.
eapply evalDist_le_1.
Qed.

Theorem evalDist_Repeat_0 :
forall (A : Set)(c : Comp A)(P : A -> bool) a,
(forall x, In x (getSupport c) -> P x = true -> x <> a) ->
evalDist (Repeat c P) a == 0.