-- Mathlib/RingTheory/Kaehler/JacobiZariski.lean
lemma Cotangent.exact :
    Function.Exact
      ((Extension.Cotangent.map (Q.toComp P).toExtensionHom).liftBaseChange T)
      (Extension.Cotangent.map (Q.ofComp P).toExtensionHom)

Proof state:
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
⊢ Extension.Cotangent.mk ⟨x, hx'⟩ = Extension.Cotangent.mk z

Tactic:
ext

Result:
case e
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
⊢ (Extension.Cotangent.mk ⟨x, hx'⟩).val = (Extension.Cotangent.mk z).val

Proof state:
case h2
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).toExtension.Cotangent
hx : (Extension.Cotangent.map (Q.ofComp P).toExtensionHom) x = 0
⊢ x ∈ LinearMap.range (LinearMap.liftBaseChange T (Extension.Cotangent.map (Q.toComp P).toExtensionHom))

Tactic:
obtain ⟨⟨x : (Q.comp P).Ring, hx'⟩, rfl⟩ := Extension.Cotangent.mk_surjective x

Result:
case h2.intro.mk
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
hx : (Extension.Cotangent.map (Q.ofComp P).toExtensionHom) (Extension.Cotangent.mk ⟨x, hx'⟩) = 0
⊢ Extension.Cotangent.mk ⟨x, hx'⟩ ∈
    LinearMap.range (LinearMap.liftBaseChange T (Extension.Cotangent.map (Q.toComp P).toExtensionHom))

Proof state:
case e_a
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : P.toExtension.Ring
hx : x ∈ P.toExtension.ker
e_2✝ : ↥Q.toExtension.ker = ↥Q.ker
⊢ ((Q.ofComp P).toExtensionHom.comp (Q.toComp P).toExtensionHom).toAlgHom =
    (IsScalarTower.toAlgHom R S Q.toExtension.Ring).comp (IsScalarTower.toAlgHom R P.Ring S)

Tactic:
refine MvPolynomial.algHom_ext fun i ↦ ?_

Result:
case e_a
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : P.toExtension.Ring
hx : x ∈ P.toExtension.ker
e_2✝ : ↥Q.toExtension.ker = ↥Q.ker
i : P.vars
⊢ ((Q.ofComp P).toExtensionHom.comp (Q.toComp P).toExtensionHom).toAlgHom (X i) =
    ((IsScalarTower.toAlgHom R S Q.toExtension.Ring).comp (IsScalarTower.toAlgHom R P.Ring S)) (X i)

Proof state:
case h2.intro.mk.intro.intro
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
this : Extension.Cotangent.mk ⟨x, hx'⟩ = Extension.Cotangent.mk z
⊢ Extension.Cotangent.mk ⟨x, hx'⟩ ∈
    Submodule.span T ↑(LinearMap.range (Extension.Cotangent.map (Q.toComp P).toExtensionHom))

Tactic:
rw [this, ← Submodule.restrictScalars_mem (Q.comp P).Ring, ← Submodule.mem_comap,
  ← Submodule.span_singleton_le_iff_mem, ← Submodule.map_le_map_iff_of_injective
  (f := Submodule.subtype _) Subtype.val_injective, Submodule.map_subtype_span_singleton,
  Submodule.span_singleton_le_iff_mem]

Result:
case h2.intro.mk.intro.intro
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
this : Extension.Cotangent.mk ⟨x, hx'⟩ = Extension.Cotangent.mk z
⊢ ↑z ∈
    Submodule.map (Submodule.subtype (Q.comp P).toExtension.ker)
      (Submodule.comap Extension.Cotangent.mk
        (Submodule.restrictScalars (Q.comp P).Ring
          (Submodule.span T ↑(LinearMap.range (Extension.Cotangent.map (Q.toComp P).toExtensionHom)))))

Proof state:
case h2.intro.mk.intro.intro
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
this : Extension.Cotangent.mk ⟨x, hx'⟩ = Extension.Cotangent.mk z
⊢ P.ker ≤
    Ideal.comap (Q.toComp P).toAlgHom.toRingHom
      (Submodule.map (Submodule.subtype (Q.comp P).toExtension.ker)
        (Submodule.comap Extension.Cotangent.mk
          (Submodule.restrictScalars (Q.comp P).Ring
            (Submodule.span T ↑(LinearMap.range (Extension.Cotangent.map (Q.toComp P).toExtensionHom))))))

Tactic:
rintro w hw

Result:
case h2.intro.mk.intro.intro
R : Type u
S : Type v
inst✝⁶ : CommRing R
inst✝⁵ : CommRing S
inst✝⁴ : Algebra R S
T : Type uT
inst✝³ : CommRing T
inst✝² : Algebra R T
inst✝¹ : Algebra S T
inst✝ : IsScalarTower R S T
Q : Generators S T
P : Generators R S
x : (Q.comp P).Ring
hx' : x ∈ (Q.comp P).toExtension.ker
y : (Q.comp P).Ring
hy : y ∈ (Q.comp P).ker * (Q.comp P).ker
e : x - y ∈ Ideal.map (Q.toComp P).toAlgHom P.ker
z : ↥(Q.comp P).ker := ⟨x - y, ⋯⟩
hz : ↑z ∈ Ideal.map (Q.toComp P).toAlgHom.toRingHom P.ker
this : Extension.Cotangent.mk ⟨x, hx'⟩ = Extension.Cotangent.mk z
w : P.Ring
hw : w ∈ P.ker
⊢ w ∈
    Ideal.comap (Q.toComp P).toAlgHom.toRingHom
      (Submodule.map (Submodule.subtype (Q.comp P).toExtension.ker)
        (Submodule.comap Extension.Cotangent.mk
          (Submodule.restrictScalars (Q.comp P).Ring
            (Submodule.span T ↑(LinearMap.range (Extension.Cotangent.map (Q.toComp P).toExtensionHom))))))