Module: Mathlib.Algebra.Category.Ring.FinitePresentation

Goal state before:
pp: case refine_2
J : Type uJ
inst✝² : Category.{vJ, uJ} J
inst✝¹ : IsFiltered J
R : CommRingCat
F : J ⥤ CommRingCat
α : (Functor.const J).obj R ⟶ F
S : CommRingCat
c : Cocone F
hc : IsColimit c
inst✝ : PreservesColimit F (forget CommRingCat)
g : S ⟶ c.pt
hc' : IsColimit ((forget CommRingCat).mapCocone c)
n : ℕ
P : CommRingCat := CommRingCat.of (MvPolynomial (Fin n) ↑R)
iP : R ⟶ P := CommRingCat.ofHom MvPolynomial.C
π : P ⟶ S
hg : ∀ (i : J), (iP ≫ π) ≫ g = α.app i ≫ c.ι.app i
this : Algebra ↑R ↑S := (CommRingCat.Hom.hom (iP ≫ π)).toAlgebra
hn : ∃ f, Function.Surjective ⇑f ∧ (ker f.toRingHom).FG
hπ : Function.Surjective ⇑(ConcreteCategory.hom π)
s : Finset ↑P
hs : Ideal.span ↑s = ker (CommRingCat.Hom.hom π)
i : J
g' : P ⟶ F.obj i
hg' : π ≫ g = g' ≫ c.ι.app i
hg'' : iP ≫ g' = α.app i
i' : ↥s → J
hi' : (r : ↥s) → i ⟶ i' r
hi'' : ∀ (r : ↥s), (ConcreteCategory.hom (F.map (hi' r))) ((ConcreteCategory.hom g') ↑r) = 0
c' : Cocone (WidePushoutShape.wideSpan i i' hi')
⊢ (iP ≫ π) ≫
      CommRingCat.ofHom
        (((CommRingCat.Hom.hom π).liftOfSurjective hπ) ⟨CommRingCat.Hom.hom (g' ≫ F.map (c'.ι.app none)), ⋯⟩) =
    α.app c'.pt

pp: case refine_3
J : Type uJ
inst✝² : Category.{vJ, uJ} J
inst✝¹ : IsFiltered J
R : CommRingCat
F : J ⥤ CommRingCat
α : (Functor.const J).obj R ⟶ F
S : CommRingCat
c : Cocone F
hc : IsColimit c
inst✝ : PreservesColimit F (forget CommRingCat)
g : S ⟶ c.pt
hc' : IsColimit ((forget CommRingCat).mapCocone c)
n : ℕ
P : CommRingCat := CommRingCat.of (MvPolynomial (Fin n) ↑R)
iP : R ⟶ P := CommRingCat.ofHom MvPolynomial.C
π : P ⟶ S
hg : ∀ (i : J), (iP ≫ π) ≫ g = α.app i ≫ c.ι.app i
this : Algebra ↑R ↑S := (CommRingCat.Hom.hom (iP ≫ π)).toAlgebra
hn : ∃ f, Function.Surjective ⇑f ∧ (ker f.toRingHom).FG
hπ : Function.Surjective ⇑(ConcreteCategory.hom π)
s : Finset ↑P
hs : Ideal.span ↑s = ker (CommRingCat.Hom.hom π)
i : J
g' : P ⟶ F.obj i
hg' : π ≫ g = g' ≫ c.ι.app i
hg'' : iP ≫ g' = α.app i
i' : ↥s → J
hi' : (r : ↥s) → i ⟶ i' r
hi'' : ∀ (r : ↥s), (ConcreteCategory.hom (F.map (hi' r))) ((ConcreteCategory.hom g') ↑r) = 0
c' : Cocone (WidePushoutShape.wideSpan i i' hi')
⊢ g =
    CommRingCat.ofHom
        (((CommRingCat.Hom.hom π).liftOfSurjective hπ) ⟨CommRingCat.Hom.hom (g' ≫ F.map (c'.ι.app none)), ⋯⟩) ≫
      c.ι.app c'.pt

Tactic:
· ext x
    suffices (iP ≫ g' ≫ F.map (c'.ι.app none)) x = α.app c'.pt x by
      simpa [RingHom.liftOfRightInverse_comp_apply] using! this
    rw [← Category.assoc, hg'', ← NatTrans.naturality]
    simp only [Functor.const_obj_obj, Functor.const_obj_map, Category.id_comp]

Goal state after:
case refine_3
J : Type uJ
inst✝² : Category.{vJ, uJ} J
inst✝¹ : IsFiltered J
R : CommRingCat
F : J ⥤ CommRingCat
α : (Functor.const J).obj R ⟶ F
S : CommRingCat
c : Cocone F
hc : IsColimit c
inst✝ : PreservesColimit F (forget CommRingCat)
g : S ⟶ c.pt
hc' : IsColimit ((forget CommRingCat).mapCocone c)
n : ℕ
P : CommRingCat := CommRingCat.of (MvPolynomial (Fin n) ↑R)
iP : R ⟶ P := CommRingCat.ofHom MvPolynomial.C
π : P ⟶ S
hg : ∀ (i : J), (iP ≫ π) ≫ g = α.app i ≫ c.ι.app i
this : Algebra ↑R ↑S := (CommRingCat.Hom.hom (iP ≫ π)).toAlgebra
hn : ∃ f, Function.Surjective ⇑f ∧ (ker f.toRingHom).FG
hπ : Function.Surjective ⇑(ConcreteCategory.hom π)
s : Finset ↑P
hs : Ideal.span ↑s = ker (CommRingCat.Hom.hom π)
i : J
g' : P ⟶ F.obj i
hg' : π ≫ g = g' ≫ c.ι.app i
hg'' : iP ≫ g' = α.app i
i' : ↥s → J
hi' : (r : ↥s) → i ⟶ i' r
hi'' : ∀ (r : ↥s), (ConcreteCategory.hom (F.map (hi' r))) ((ConcreteCategory.hom g') ↑r) = 0
c' : Cocone (WidePushoutShape.wideSpan i i' hi')
⊢ g =
    CommRingCat.ofHom
        (((CommRingCat.Hom.hom π).liftOfSurjective hπ) ⟨CommRingCat.Hom.hom (g' ≫ F.map (c'.ι.app none)), ⋯⟩) ≫
      c.ι.app c'.pt