-- Mathlib/RingTheory/Unramified/Basic.lean
theorem lift_unique
    [FormallyUnramified R A] (I : Ideal B) (hI : IsNilpotent I) (g₁ g₂ : A →ₐ[R] B)
    (h : (Ideal.Quotient.mkₐ R I).comp g₁ = (Ideal.Quotient.mkₐ R I).comp g₂) : g₁ = g₂

Proof state:
case h₂
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
e : (Ideal.Quotient.mkₐ R J).comp g₁ = (Ideal.Quotient.mkₐ R J).comp g₂
⊢ g₁ = g₂

Tactic:
apply h₁

Result:
case h₂.a
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
e : (Ideal.Quotient.mkₐ R J).comp g₁ = (Ideal.Quotient.mkₐ R J).comp g₂
⊢ (Ideal.Quotient.mkₐ R I).comp g₁ = (Ideal.Quotient.mkₐ R I).comp g₂

Proof state:
case h₂.a.a.H
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
x : A
e : ((Ideal.Quotient.mkₐ R J).comp g₁) x = ((Ideal.Quotient.mkₐ R J).comp g₂) x
⊢ ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₁)) x =
    ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₂)) x

Tactic:
dsimp only [AlgHom.comp_apply, Ideal.Quotient.mkₐ_eq_mk] at e ⊢

Result:
case h₂.a.a.H
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
x : A
e : (Ideal.Quotient.mk J) (g₁ x) = (Ideal.Quotient.mk J) (g₂ x)
⊢ (Ideal.Quotient.mk (Ideal.map (Ideal.Quotient.mk I) J)) ((Ideal.Quotient.mk I) (g₁ x)) =
    (Ideal.Quotient.mk (Ideal.map (Ideal.Quotient.mk I) J)) ((Ideal.Quotient.mk I) (g₂ x))

Proof state:
R : Type v
inst✝⁵ : CommRing R
A : Type u
inst✝⁴ : CommRing A
inst✝³ : Algebra R A
B : Type w
inst✝² : CommRing B
inst✝¹ : Algebra R B
inst✝ : FormallyUnramified R A
I : Ideal B
hI : IsNilpotent I
⊢ ∀ (g₁ g₂ : A →ₐ[R] B), (Ideal.Quotient.mkₐ R I).comp g₁ = (Ideal.Quotient.mkₐ R I).comp g₂ → g₁ = g₂

Tactic:
change Function.Injective (Ideal.Quotient.mkₐ R I).comp

Result:
R : Type v
inst✝⁵ : CommRing R
A : Type u
inst✝⁴ : CommRing A
inst✝³ : Algebra R A
B : Type w
inst✝² : CommRing B
inst✝¹ : Algebra R B
inst✝ : FormallyUnramified R A
I : Ideal B
hI : IsNilpotent I
⊢ Function.Injective (Ideal.Quotient.mkₐ R I).comp

Proof state:
R : Type v
inst✝⁵ : CommRing R
A : Type u
inst✝⁴ : CommRing A
inst✝³ : Algebra R A
B : Type w
inst✝² : CommRing B
inst✝¹ : Algebra R B
inst✝ : FormallyUnramified R A
I : Ideal B
hI : IsNilpotent I
g₁ g₂ : A →ₐ[R] B
h : (Ideal.Quotient.mkₐ R I).comp g₁ = (Ideal.Quotient.mkₐ R I).comp g₂
⊢ g₁ = g₂

Tactic:
revert g₁ g₂

Result:
R : Type v
inst✝⁵ : CommRing R
A : Type u
inst✝⁴ : CommRing A
inst✝³ : Algebra R A
B : Type w
inst✝² : CommRing B
inst✝¹ : Algebra R B
inst✝ : FormallyUnramified R A
I : Ideal B
hI : IsNilpotent I
⊢ ∀ (g₁ g₂ : A →ₐ[R] B), (Ideal.Quotient.mkₐ R I).comp g₁ = (Ideal.Quotient.mkₐ R I).comp g₂ → g₁ = g₂

Proof state:
case h₂.a.a
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
e : (Ideal.Quotient.mkₐ R J).comp g₁ = (Ideal.Quotient.mkₐ R J).comp g₂
⊢ (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₁) =
    (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₂)

Tactic:
ext x

Result:
case h₂.a.a.H
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
e : (Ideal.Quotient.mkₐ R J).comp g₁ = (Ideal.Quotient.mkₐ R J).comp g₂
x : A
⊢ ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₁)) x =
    ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₂)) x

Proof state:
case h₂.a.a.H
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
e : (Ideal.Quotient.mkₐ R J).comp g₁ = (Ideal.Quotient.mkₐ R J).comp g₂
x : A
⊢ ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₁)) x =
    ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₂)) x

Tactic:
replace e := AlgHom.congr_fun e x

Result:
case h₂.a.a.H
R : Type v
inst✝⁶ : CommRing R
A : Type u
inst✝⁵ : CommRing A
inst✝⁴ : Algebra R A
B✝ : Type w
inst✝³ : CommRing B✝
inst✝² : FormallyUnramified R A
I✝ : Ideal B✝
hI : IsNilpotent I✝
B : Type w
inst✝¹ : CommRing B
I J : Ideal B
hIJ : I ≤ J
h₁ : ∀ [inst : Algebra R B], Function.Injective (Ideal.Quotient.mkₐ R I).comp
h₂ : ∀ [inst : Algebra R (B ⧸ I)], Function.Injective (Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp
inst✝ : Algebra R B
g₁ g₂ : A →ₐ[R] B
x : A
e : ((Ideal.Quotient.mkₐ R J).comp g₁) x = ((Ideal.Quotient.mkₐ R J).comp g₂) x
⊢ ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₁)) x =
    ((Ideal.Quotient.mkₐ R (Ideal.map (Ideal.Quotient.mk I) J)).comp ((Ideal.Quotient.mkₐ R I).comp g₂)) x