-- Mathlib/Topology/Instances/Complex.lean
theorem Complex.uniformContinuous_ringHom_eq_id_or_conj (K : Subfield ℂ) {ψ : K →+* ℂ}
    (hc : UniformContinuous ψ) : ψ.toFun = K.subtype ∨ ψ.toFun = conj ∘ K.subtype

Proof state:
case refine_1
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
⊢ Function.Surjective j

Tactic:
rintro ⟨y, hy⟩

Result:
case refine_1.mk
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
y : ℂ
hy : y ∈ ↑K.topologicalClosure
⊢ ∃ a, j a = ⟨y, hy⟩

Proof state:
case refine_2.inl.h.h.intro
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝¹ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this✝ : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ofRealHom.fieldRange
j : ↥K.topologicalClosure ≃+* ↥ofRealHom.fieldRange := RingEquiv.subfieldCongr h
ψ₁ : ℝ →+* ℂ := extψ.comp (j.symm.toRingHom.comp ofRealHom.rangeRestrict)
hψ₁ : Continuous ⇑ψ₁
x : ↥K
r : ℝ
hr : ofRealHom.rangeRestrict r = j (ι x)
this : extψ (↑j.symm (j (ι x))) = ofRealHom r
⊢ (↑↑ψ).toFun x = K.subtype x

Tactic:
convert this using 1

Result:
case h.e'_2
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝¹ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this✝ : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ofRealHom.fieldRange
j : ↥K.topologicalClosure ≃+* ↥ofRealHom.fieldRange := RingEquiv.subfieldCongr h
ψ₁ : ℝ →+* ℂ := extψ.comp (j.symm.toRingHom.comp ofRealHom.rangeRestrict)
hψ₁ : Continuous ⇑ψ₁
x : ↥K
r : ℝ
hr : ofRealHom.rangeRestrict r = j (ι x)
this : extψ (↑j.symm (j (ι x))) = ofRealHom r
⊢ (↑↑ψ).toFun x = extψ (↑j.symm (j (ι x)))

case h.e'_3
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝¹ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this✝ : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ofRealHom.fieldRange
j : ↥K.topologicalClosure ≃+* ↥ofRealHom.fieldRange := RingEquiv.subfieldCongr h
ψ₁ : ℝ →+* ℂ := extψ.comp (j.symm.toRingHom.comp ofRealHom.rangeRestrict)
hψ₁ : Continuous ⇑ψ₁
x : ↥K
r : ℝ
hr : ofRealHom.rangeRestrict r = j (ι x)
this : extψ (↑j.symm (j (ι x))) = ofRealHom r
⊢ K.subtype x = ofRealHom r

Proof state:
case refine_2.inl.h
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ofRealHom.fieldRange
j : ↥K.topologicalClosure ≃+* ↥ofRealHom.fieldRange := RingEquiv.subfieldCongr h
⊢ (↑↑ψ).toFun = ⇑K.subtype

Tactic:
let ψ₁ := RingHom.comp extψ (RingHom.comp j.symm.toRingHom ofRealHom.rangeRestrict)

Result:
case refine_2.inl.h
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ofRealHom.fieldRange
j : ↥K.topologicalClosure ≃+* ↥ofRealHom.fieldRange := RingEquiv.subfieldCongr h
ψ₁ : ℝ →+* ℂ := extψ.comp (j.symm.toRingHom.comp ofRealHom.rangeRestrict)
⊢ (↑↑ψ).toFun = ⇑K.subtype

Proof state:
case h.e'_4
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
x : { x // x ∈ closure (id '' {x | ↑K x}) }
⊢ ↑K.topologicalClosure = closure {x | ↑K x}

Tactic:
rfl

Result:
no goals