induction ls; intuition.
unfold sumList; simpl.
symmetry.
eapply ratMult_0_l.
rewrite sumList_cons.
simpl in *.
rewrite H; intuition.
rewrite IHls; intuition.
rewrite ratMult_comm.
rewrite ratMult_ratAdd_cd.
simpl.
eapply ratMult_comm.
Qed.

(* Specifications: *)
Theorem filter_app :
forall (A : Set)(ls1 ls2 : list A)(f : A -> bool),
filter f (ls1 ++ ls2) = filter f ls1 ++ filter f ls2.