induction ls; intuition; simpl in *.
unfold sumList.
simpl.
eapply ratAdd_0_l.
unfold sumList in *.
simpl in *.
erewrite fold_add_body_eq.
2:{
eapply ratAdd_comm.
}
2:{
intuition.
eapply eqRat_refl.
}
rewrite fold_add_init.
destruct (P a);
simpl.
erewrite (fold_add_body_eq (filter P ls)).
2:{
eapply ratAdd_comm.
}
2:{
intuition.
eapply eqRat_refl.
}
rewrite fold_add_init.
rewrite ratAdd_assoc.
eapply ratAdd_eqRat_compat;
intuition.
erewrite (fold_add_body_eq (filter (fun a => negb (P a)) ls)).
2:{
eapply ratAdd_comm.
}
2:{
intuition.
eapply eqRat_refl.
}
rewrite fold_add_init.
rewrite <- ratAdd_assoc.
rewrite (ratAdd_comm (fold_left (fun (a0 : Rat) (b : A) => a0 + f b) (filter P ls) 0 )).
rewrite ratAdd_assoc.
eapply ratAdd_eqRat_compat;
intuition.
Qed.

Lemma sumList_rel_sumList_eqRat : forall (A : Set)(ls : list A)(f : A -> Rat),
sumList_rel (fun a r => f a == r) ls
(sumList ls f).