(* This is the proof found at http://planetmath.org/ProofOfLimitRuleOfProduct.html *)
unfold rat_inf_limit, inf_limit in *.
intuition.
edestruct (H1 ((ratInverse (1 + c2)) * epsilon * (1/ 2))).
intuition.
apply ratMult_0 in H4.
intuition.
apply ratMult_0 in H5.
intuition.
eapply ratInverse_nz.
eauto.
edestruct (H2 ((ratInverse (1 + c1)) * epsilon * (1 / 2))).
intuition.
apply ratMult_0 in H5.
intuition.
apply ratMult_0 in H6.
intuition.
eapply ratInverse_nz.
eauto.
edestruct (H2 1).
intuition.
exists (maxList (x :: x0 :: x1 :: nil)).
intuition.
unfold ratMult_rel in *.
edestruct H.
edestruct H0.
rewrite H8; eauto.
eapply leRat_trans.
eapply (ratTriangleInequality _ _ (c1 * x3)).
rewrite ratMult_ratDistance_factor_r.
rewrite ratMult_ratDistance_factor_l.
rewrite ratMult_comm.
eapply leRat_trans.
eapply ratAdd_leRat_compat.
eapply ratMult_leRat_compat.
eapply ratDistance_le_sum.
eapply H6; [idtac | eapply H10].
eapply le_trans.
eapply (maxList_correct (x :: x0 :: x1 :: nil)).
simpl.
intuition.
trivial.
eapply H4; [idtac | eapply H9].
eapply le_trans.
eapply (maxList_correct (x :: x0 :: x1 :: nil)).
simpl.
intuition.
trivial.
eapply ratMult_leRat_compat.
eapply (ratAdd_any_leRat_l 1).
eapply leRat_refl.
eapply H5; [idtac | eapply H10].
eapply le_trans.
eapply (maxList_correct (x :: x0 :: x1 :: nil)).
simpl.
intuition.
trivial.
eapply eqRat_impl_leRat.
repeat rewrite <- ratMult_assoc.
rewrite (ratAdd_comm c2).
rewrite (ratAdd_comm c1).
rewrite (ratMult_assoc ((1 + c2) * (ratInverse (1 + c2)))).
rewrite (ratMult_assoc ((1 + c1) * (ratInverse (1 + c1)))).
rewrite <- ratMult_distrib_r.
rewrite (ratMult_comm (1 + c2)).
rewrite (ratMult_comm (1 + c1)).
repeat rewrite ratInverse_prod_1.
rewrite ratMult_2.
rewrite ratMult_1_l.
rewrite ratMult_comm.
rewrite ratMult_assoc.
rewrite ratMult_eq_rat1.
eapply ratMult_1_r.
intuition.
assert (1 <= 0).
eapply leRat_trans.
eapply (ratAdd_any_leRat_r c1).
eapply leRat_refl.
eapply eqRat_impl_leRat.
rewrite ratAdd_comm.
trivial.
intuition.
intuition.
assert (1 <= 0).
eapply leRat_trans.
eapply (ratAdd_any_leRat_r c2).
eapply leRat_refl.
eapply eqRat_impl_leRat.
rewrite ratAdd_comm.
trivial.
intuition.
Qed.
Definition rat_limit(f : Rat -> Rat -> Prop)(p L : Rat) :=
forall epsilon,
~ (epsilon == 0) ->
exists delta, (~delta == 0) /\
forall x v,
f x v ->
(ratDistance x p) <= delta ->
(ratDistance v L) <= epsilon.
Definition continuous_at(f : Rat -> Rat -> Prop)(c : Rat) :=
forall v, f c v ->
rat_limit f c v.

Lemma rat_inf_limit_comp : forall (f : nat -> Rat -> Prop)(g : Rat -> Rat -> Prop) a v,
rat_inf_limit f a ->
g a v ->
continuous_at g a ->
left_total f ->
rat_inf_limit (fun n r => forall v', (f n v' -> g v' r)) v.