intuition.
unfold DistSingle_Adv.
rewrite G_hybrid_DistSingle_eq.
rewrite G_hybrid_DistSingle_S_eq.
eapply ratDistance_comm.
Qed.

Theorem ratDistance_sequence_sum :
forall (f : nat -> Rat)(n : nat),
| f O - f n | <= sumList (forNats n) (fun i => | f i - f (S i) |).