-- Lean 4 proof step from Mathlib.Condensed.Light.Sequence (goal)
Goal:
R : Type
inst : CommRing R
X Y : LightCondMod R
p : X ⟶ Y
hp : Epi p
S : LightProfinite
f :
  obj (free R)
      (tensorObj (𝒞 :=
          CompHausLike.category fun (X : TopCat.{0}) ↦
            TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
          S ℕ∪{∞}).toCondensed ⟶
    Y
T : LightProfinite
π :
  T ⟶
    tensorObj (𝒞 :=
      CompHausLike.category fun (X : TopCat.{0}) ↦
        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
      S ℕ∪{∞}
g : obj (free R) T.toCondensed ⟶ X
hπ : Epi π
comm : CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) π ≫ f = g ≫ p
S' T' : LightProfinite
y' : S' ⟶ S
π' :
  T' ⟶
    tensorObj (𝒞 :=
      CompHausLike.category fun (X : TopCat.{0}) ↦
        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
      S' ℕ∪{∞}
g' : T' ⟶ T
hπ'_1 : Epi π'
hy' : Epi y'
comp : π' ≫ y' ▷ ℕ∪{∞} = g' ≫ π
epi : Epi (cover π')
split :
  SplitEpi (X := LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞}))
    (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}) ≫ π' ≫ fst S' ℕ∪{∞})
hS' :
  Nonempty
    (↑(S'.toTop (P := fun (X : TopCat.{0}) ↦
          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
      Type)
this_1 : Mono (X := LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞})) (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}))
this_2 :
  Nonempty
    (↑((LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞})).toTop (P := fun (X : TopCat.{0}) ↦
          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
      Type)
r_inf : T' ⟶ LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞})
hr : LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}) ≫ r_inf = 𝟙 (LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞}))
hπ'' : RegularEpi (lightProfiniteToLightCondSet.map π') := regularEpiOfEpi (lightProfiniteToLightCondSet.map π')
this : EffectiveEpi π'
hπ' : RegularEpi (lightProfiniteToLightCondSet.map π') :=
  regularEpiOfPreserves π' lightProfiniteToLightCondSet
    (pullback.cone (P := fun (X : TopCat.{0}) ↦
      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π' π')
    (pullback.isLimit (P := fun (X : TopCat.{0}) ↦
      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π' π')
hc :
  Limits.IsColimit (C := LightCondMod R) (F := Limits.parallelPair hπ'.left hπ'.right ⋙ free R)
    (mapCocone (free R) (Limits.Cofork.ofπ (lightProfiniteToLightCondSet.map π') ⋯))
⊢ Eq (α := obj (Limits.parallelPair hπ'.left hπ'.right ⋙ free R) Limits.WalkingParallelPair.one ⟶ Y)
    (CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) (π' ≫ y' ▷ ℕ∪{∞}) ≫ f :
      obj (lightProfiniteToLightCondSet ⋙ free R) T' ⟶ Y)
    (NatTrans.app
        (Limits.Cocone.ι
          (cocone R (X := X) π' (CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) g' ≫ g) r_inf
            split.section_ hr))
        Limits.WalkingParallelPair.one ≫
      p)
have pos : CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map π') ≫
    CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map (y' ▷ ℕ∪{∞})) ≫ f =
  CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map g') ≫
        CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map π) ≫ f -
      CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map r_inf) ≫
        CategoryTheory.Functor.map (free R)
            (lightProfiniteToLightCondSet.map (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}))) ≫
          CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map g') ≫
            CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map π) ≫ f +
    CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map r_inf) ≫
      CategoryTheory.Functor.map (free R)
          (lightProfiniteToLightCondSet.map (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}))) ≫
        CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map π') ≫
          CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map (fst S' ℕ∪{∞})) ≫
            CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map split.section_) ≫
              CategoryTheory.Functor.map (free R)
                  (lightProfiniteToLightCondSet.map (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}))) ≫
                CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map g') ≫
                  CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map π) ≫ f
Tactic: simp only [comp_obj, Limits.parallelPair_obj_one, Functor.comp_map, Functor.map_comp,
      assoc, cocone_pt, cocone_ι_app, eqToHom_refl, Preadditive.comp_add, Preadditive.comp_sub,
      id_comp, Preadditive.add_comp, Preadditive.sub_comp, ← comm]