induction ls; intuition; unfold sumList in *; simpl in *.
exfalso.
apply H.
eapply eqRat_refl.
destruct H.
intuition.
assert (f a + fold_left (fun r a => r + f a) ls 0 == 0 -> False).
intuition.
eapply H.
eapply eqRat_trans.
eapply fold_add_body_eq.
eapply ratAdd_comm.
intuition.
eapply eqRat_refl.
eapply eqRat_trans.
eapply fold_add_init.
trivial.
eapply ratAdd_nz in H0.
destruct H0.
econstructor; intuition.
apply IHls in H0.
destruct H0; intuition.
exists x.
intuition.
assert (f a + fold_left (fun r a => r + f a) ls 0 == 0).
eapply eqRat_trans.
eapply eqRat_symm.
eapply fold_add_init.
eapply eqRat_trans.
eapply fold_add_body_eq.
eapply ratAdd_comm.
intuition.
eapply eqRat_refl.
trivial.
apply ratAdd_0 in H1.
intuition.
destruct H.
intuition.
subst.
intuition.
apply IHls in H3; intuition.
econstructor; eauto.
Qed.
End SumList.

Lemma fold_add_init_0 : forall (A : Set)(ls : list A) f init,
fold_left (fun r a => r + (f a)) ls init == init + fold_left (fun r a => r + (f a)) ls 0.