/-- **Fundamental theorem of calculus-1**, strict derivative in both limits for a locally finite
measure.

Let `f` be a measurable function integrable on `a..b`. Let `(la, la')` be a pair of
`intervalIntegral.FTCFilter`s around `a`; let `(lb, lb')` be a pair of `intervalIntegral.FTCFilter`s
around `b`. Suppose that `f` has finite limits `ca` and `cb` at `la' ⊓ μ.ae` and `lb' ⊓ μ.ae`,
respectively.
Then `∫ x in va..vb, f x ∂μ - ∫ x in ua..ub, f x ∂μ =
  ∫ x in ub..vb, cb ∂μ - ∫ x in ua..va, ca ∂μ +
    o(‖∫ x in ua..va, (1:ℝ) ∂μ‖ + ‖∫ x in ub..vb, (1:ℝ) ∂μ‖)`
as `ua` and `va` tend to `la` while `ub` and `vb` tend to `lb`.
-/
theorem measure_integral_sub_integral_sub_linear_isLittleO_of_tendsto_ae
    (hab : IntervalIntegrable f μ a b) (hmeas_a : StronglyMeasurableAtFilter f la' μ)
    (hmeas_b : StronglyMeasurableAtFilter f lb' μ) (ha_lim : Tendsto f (la' ⊓ μ.ae) (𝓝 ca))
    (hb_lim : Tendsto f (lb' ⊓ μ.ae) (𝓝 cb)) (hua : Tendsto ua lt la) (hva : Tendsto va lt la)
    (hub : Tendsto ub lt lb) (hvb : Tendsto vb lt lb) :
    (fun t =>
        ((∫ x in va t..vb t, f x ∂μ) - ∫ x in ua t..ub t, f x ∂μ) -
          ((∫ _ in ub t..vb t, cb ∂μ) - ∫ _ in ua t..va t, ca ∂μ)) =o[lt]
      fun t => ‖∫ _ in ua t..va t, (1 : ℝ) ∂μ‖ + ‖∫ _ in ub t..vb t, (1 : ℝ) ∂μ‖ := by
  haveI := FTCFilter.meas_gen la; haveI := FTCFilter.meas_gen lb
  refine'
    ((measure_integral_sub_linear_isLittleO_of_tendsto_ae hmeas_a ha_lim hua hva).neg_left.add_add
          (measure_integral_sub_linear_isLittleO_of_tendsto_ae hmeas_b hb_lim hub hvb)).congr'
      _ EventuallyEq.rfl
  have A : ∀ᶠ t in lt, IntervalIntegrable f μ (ua t) (va t) :=
    ha_lim.eventually_intervalIntegrable_ae hmeas_a (FTCFilter.finiteAt_inner la) hua hva
  have A' : ∀ᶠ t in lt, IntervalIntegrable f μ a (ua t) :=
    ha_lim.eventually_intervalIntegrable_ae hmeas_a (FTCFilter.finiteAt_inner la)
      (tendsto_const_pure.mono_right FTCFilter.pure_le) hua
  have B : ∀ᶠ t in lt, IntervalIntegrable f μ (ub t) (vb t) :=
    hb_lim.eventually_intervalIntegrable_ae hmeas_b (FTCFilter.finiteAt_inner lb) hub hvb
  have B' : ∀ᶠ t in lt, IntervalIntegrable f μ b (ub t) :=
    hb_lim.eventually_intervalIntegrable_ae hmeas_b (FTCFilter.finiteAt_inner lb)
      (tendsto_const_pure.mono_right FTCFilter.pure_le) hub
  filter_upwards [A, A', B, B'] with _ ua_va a_ua ub_vb b_ub
  

Proof state:
case h
ι : Type u_1
𝕜 : Type u_2
E : Type u_3
F : Type u_4
A✝ : Type u_5
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
f : ℝ → E
a b : ℝ
c ca cb : E
l l' la la' lb lb' : Filter ℝ
lt : Filter ι
μ : Measure ℝ
u v ua va ub vb : ι → ℝ
inst✝³ : CompleteSpace E
inst✝² : FTCFilter a la la'
inst✝¹ : FTCFilter b lb lb'
inst✝ : IsLocallyFiniteMeasure μ
hab : IntervalIntegrable f μ a b
hmeas_a : StronglyMeasurableAtFilter f la'
hmeas_b : StronglyMeasurableAtFilter f lb'
ha_lim : Tendsto f (la' ⊓ Measure.ae μ) (𝓝 ca)
hb_lim : Tendsto f (lb' ⊓ Measure.ae μ) (𝓝 cb)
hua : Tendsto ua lt la
hva : Tendsto va lt la
hub : Tendsto ub lt lb
hvb : Tendsto vb lt lb
this✝ : IsMeasurablyGenerated la'
this : IsMeasurablyGenerated lb'
A : ∀ᶠ (t : ι) in lt, IntervalIntegrable f μ (ua t) (va t)
A' : ∀ᶠ (t : ι) in lt, IntervalIntegrable f μ a (ua t)
B : ∀ᶠ (t : ι) in lt, IntervalIntegrable f μ (ub t) (vb t)
B' : ∀ᶠ (t : ι) in lt, IntervalIntegrable f μ b (ub t)
a✝ : ι
ua_va : IntervalIntegrable f μ (ua a✝) (va a✝)
a_ua : IntervalIntegrable f μ a (ua a✝)
ub_vb : IntervalIntegrable f μ (ub a✝) (vb a✝)
b_ub : IntervalIntegrable f μ b (ub a✝)
⊢ -(∫ (x : ℝ) in ua a✝..va a✝, f x ∂μ - ∫ (x : ℝ) in ua a✝..va a✝, ca ∂μ) +
      (∫ (x : ℝ) in ub a✝..vb a✝, f x ∂μ - ∫ (x : ℝ) in ub a✝..vb a✝, cb ∂μ) =
    ∫ (x : ℝ) in va a✝..vb a✝, f x ∂μ - ∫ (x : ℝ) in ua a✝..ub a✝, f x ∂μ -
      (∫ (x : ℝ) in ub a✝..vb a✝, cb ∂μ - ∫ (x : ℝ) in ua a✝..va a✝, ca ∂μ)

Next tactic:
rw [← integral_interval_sub_interval_comm']