-- Mathlib/MeasureTheory/OuterMeasure/Induced.lean
theorem extend_mono {s₁ s₂ : Set α} (h₁ : MeasurableSet s₁) (hs : s₁ ⊆ s₂) :
    extend m s₁ ≤ extend m s₂

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m (s₁ ∪ s₂ \ s₁) = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ m s₂ h₂

Tactic:
rw [union_diff_cancel hs] at this

Result:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m s₂ = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ m s₂ h₂

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m s₂ = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ m s₂ h₂

Tactic:
rw [← extend_eq m]

Result:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m s₂ = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ extend m s₂

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
⊢ extend m s₁ ≤ extend m s₂

Tactic:
refine le_iInf ?_

Result:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
⊢ ∀ (i : MeasurableSet s₂), extend m s₁ ≤ m s₂ i

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m s₂ = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ extend m s₂

Tactic:
exact le_iff_exists_add.2 ⟨_, this⟩

Result:
no goals

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
⊢ extend m s₁ ≤ m s₂ h₂

Tactic:
have :=
  extend_union MeasurableSet.empty m0 MeasurableSet.iUnion mU disjoint_sdiff_self_right h₁
    (h₂.diff h₁)

Result:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
this : extend m (s₁ ∪ s₂ \ s₁) = extend m s₁ + extend m (s₂ \ s₁)
⊢ extend m s₁ ≤ m s₂ h₂

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
⊢ ∀ (i : MeasurableSet s₂), extend m s₁ ≤ m s₂ i

Tactic:
intro h₂

Result:
α : Type u_1
inst✝ : MeasurableSpace α
m : (s : Set α) → MeasurableSet s → ℝ≥0∞
m0 : m ∅ ⋯ = 0
mU :
  ∀ ⦃f : ℕ → Set α⦄ (hm : ∀ (i : ℕ), MeasurableSet (f i)),
    Pairwise (Disjoint on f) → m (⋃ i, f i) ⋯ = ∑' (i : ℕ), m (f i) ⋯
s₁ s₂ : Set α
h₁ : MeasurableSet s₁
hs : s₁ ⊆ s₂
h₂ : MeasurableSet s₂
⊢ extend m s₁ ≤ m s₂ h₂