-- Mathlib/Topology/MetricSpace/Completion.lean
theorem mem_uniformity_dist (s : Set (Completion α × Completion α)) :
    s ∈ 𝓤 (Completion α) ↔ ∃ ε > 0, ∀ {a b}, dist a b < ε → (a, b) ∈ s

Proof state:
case mp.intro.intro.intro.intro.intro
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
this : ε ≤ dist x y ∨ (x, y) ∈ t
⊢ (x, y) ∈ s

Tactic:
simp only [not_le.mpr hxy, false_or, not_le] at this

Result:
case mp.intro.intro.intro.intro.intro
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
this : (x, y) ∈ t
⊢ (x, y) ∈ s

Proof state:
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
⊢ ε ≤ dist x y ∨ (x, y) ∈ t

Tactic:
refine induction_on₂ x y ?_ ?_

Result:
case refine_1
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
⊢ IsClosed {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t}

case refine_2
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
⊢ ∀ (a b : α), ε ≤ dist ↑a ↑b ∨ (↑a, ↑b) ∈ t

Proof state:
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
ε : ℝ
εpos : ε > 0
hε : ∀ {a b : Completion α}, dist a b < ε → (a, b) ∈ s
r : Set (ℝ × ℝ) := {p | dist p.1 p.2 < ε}
this✝ : r ∈ 𝓤 ℝ
t1 : Set (Completion α × Completion α)
ht1 : t1 ∈ 𝓤 (Completion α)
t2 : Set (Completion α × Completion α)
ht2 : t2 ∈ 𝓤 (Completion α)
ht : t1 ×ˢ t2 ⊆ (fun p => ((p.1.1, p.2.1), p.1.2, p.2.2)) ⁻¹' {x | (dist x.1.1 x.1.2, dist x.2.1 x.2.2) ∈ r}
a b : Completion α
hab : (a, b) ∈ t1
this : ((a, b), a, a) ∈ t1 ×ˢ t2
I : ((a, b), a, a) ∈ (fun p => ((p.1.1, p.2.1), p.1.2, p.2.2)) ⁻¹' {x | (dist x.1.1 x.1.2, dist x.2.1 x.2.2) ∈ r}
⊢ dist a b < ε

Tactic:
simp? [r, Completion.dist_self, Real.dist_eq, Completion.dist_comm] at I says
  simp only [Real.dist_eq, mem_setOf_eq, preimage_setOf_eq, Completion.dist_self,
    Completion.dist_comm, zero_sub, abs_neg, r] at I

Result:
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
ε : ℝ
εpos : ε > 0
hε : ∀ {a b : Completion α}, dist a b < ε → (a, b) ∈ s
r : Set (ℝ × ℝ) := {p | dist p.1 p.2 < ε}
this✝ : r ∈ 𝓤 ℝ
t1 : Set (Completion α × Completion α)
ht1 : t1 ∈ 𝓤 (Completion α)
t2 : Set (Completion α × Completion α)
ht2 : t2 ∈ 𝓤 (Completion α)
ht : t1 ×ˢ t2 ⊆ (fun p => ((p.1.1, p.2.1), p.1.2, p.2.2)) ⁻¹' {x | (dist x.1.1 x.1.2, dist x.2.1 x.2.2) ∈ r}
a b : Completion α
hab : (a, b) ∈ t1
this : ((a, b), a, a) ∈ t1 ×ˢ t2
I : |dist a b| < ε
⊢ dist a b < ε

Proof state:
case refine_1
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
this : {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t} = {p | ε ≤ dist p.1 p.2} ∪ t
⊢ IsClosed {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t}

Tactic:
rw [this]

Result:
case refine_1
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
this : {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t} = {p | ε ≤ dist p.1 p.2} ∪ t
⊢ IsClosed ({p | ε ≤ dist p.1 p.2} ∪ t)

Proof state:
case neg
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x✝ y✝ : Completion α
hxy : dist x✝ y✝ < ε
x y : α
h : ¬ε ≤ dist x y
Z : (x, y) ∈ {x | (↑x.1, ↑x.2) ∈ t}
⊢ ε ≤ dist x y ∨ (↑x, ↑y) ∈ t

Tactic:
simp only [Set.mem_setOf_eq] at Z

Result:
case neg
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x✝ y✝ : Completion α
hxy : dist x✝ y✝ < ε
x y : α
h : ¬ε ≤ dist x y
Z : (↑x, ↑y) ∈ t
⊢ ε ≤ dist x y ∨ (↑x, ↑y) ∈ t

Proof state:
case neg
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x✝ y✝ : Completion α
hxy : dist x✝ y✝ < ε
x y : α
h : ¬ε ≤ dist x y
Z : (↑x, ↑y) ∈ t
⊢ ε ≤ dist x y ∨ (↑x, ↑y) ∈ t

Tactic:
exact Or.inr Z

Result:
no goals

Proof state:
case h
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
x✝ : Completion α × Completion α
⊢ x✝ ∈ {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t} ↔ x✝ ∈ {p | ε ≤ dist p.1 p.2} ∪ t

Tactic:
simp

Result:
no goals

Proof state:
case refine_1
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
⊢ IsClosed {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t}

Tactic:
have : { x : Completion α × Completion α | ε ≤ dist x.fst x.snd ∨ (x.fst, x.snd) ∈ t } =
       { p : Completion α × Completion α | ε ≤ dist p.1 p.2 } ∪ t := by ext; simp

Result:
case refine_1
α : Type u
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
hs : s ∈ 𝓤 (Completion α)
t : Set (Completion α × Completion α)
ht : t ∈ 𝓤 (Completion α)
tclosed : IsClosed t
ts : t ⊆ s
A : {x | (↑x.1, ↑x.2) ∈ t} ∈ 𝓤 α
ε : ℝ
εpos : ε > 0
hε : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ {x | (↑x.1, ↑x.2) ∈ t}
x y : Completion α
hxy : dist x y < ε
this : {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t} = {p | ε ≤ dist p.1 p.2} ∪ t
⊢ IsClosed {x | ε ≤ dist x.1 x.2 ∨ (x.1, x.2) ∈ t}