-- Lean 4 proof step from Mathlib.Condensed.Light.Sequence (goal)
Goal:
R : Type
inst : CommRing R
X : LightCondMod R
S T : LightProfinite
π :
  T ⟶
    tensorObj (𝒞 :=
      CompHausLike.category fun (X : TopCat.{0}) ↦
        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
      S ℕ∪{∞}
inst_1 :
  Epi (C := LightCondMod R) (X :=
    obj (lightProfiniteToLightCondSet ⋙ free R)
      (CompHausLike.of
        (fun (X : TopCat.{0}) ↦ TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
        ((↑(T.toTop (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type) ⊕
          (↑((CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                    LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                    (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                    (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)).toTop
                (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type))))
    (CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) (cover π))
g : obj (lightProfiniteToLightCondSet ⋙ free R) T ⟶ X
r_inf : T ⟶ LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})
σ : S ⟶ LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})
hr : LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ r_inf = 𝟙 (LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
⊢ Eq (α :=
    obj
        (pair T
            (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
              TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
              LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
              (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π) (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)) ⋙
          lightProfiniteToLightCondSet ⋙ free R)
        { as := WalkingPair.left : Discrete WalkingPair } ⟶
      X)
    (CategoryStruct.comp (Y :=
      obj
        (obj (Functor.const (Discrete WalkingPair))
          (Cocone.pt (F :=
            pair T
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                  LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)) ⋙
              lightProfiniteToLightCondSet ⋙ free R)
            (mapCocone (lightProfiniteToLightCondSet ⋙ free R)
              (coproductCocone (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                  LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))))
        { as := WalkingPair.left : Discrete WalkingPair })
      (NatTrans.app
        (Cocone.ι
          (mapCocone (lightProfiniteToLightCondSet ⋙ free R)
            (coproductCocone (P := fun (X : TopCat.{0}) ↦
              TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
              (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)))))
        { as := WalkingPair.left })
      (CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) (cover π) ≫
        CategoryTheory.Functor.map
            (parallelPair (X :=
                lightProfiniteToLightCondSet.obj
                  (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
                (lightProfiniteToLightCondSet.map
                  (CompHausLike.pullback.fst (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
                (lightProfiniteToLightCondSet.map
                  (CompHausLike.pullback.snd (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π)) ⋙
              free R)
            WalkingParallelPairHom.left ≫
          (g -
              CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R)
                  (r_inf ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞})) ≫
                g +
            CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R)
                (r_inf ≫
                  LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫
                    π ≫ fst S ℕ∪{∞} ≫ σ ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞})) ≫
              g)))
    (CategoryStruct.comp (Y :=
      obj
        (obj (Functor.const (Discrete WalkingPair))
          (Cocone.pt (F :=
            pair T
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                  LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)) ⋙
              lightProfiniteToLightCondSet ⋙ free R)
            (mapCocone (lightProfiniteToLightCondSet ⋙ free R)
              (coproductCocone (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                  LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                  (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))))
        { as := WalkingPair.left : Discrete WalkingPair })
      (NatTrans.app
        (Cocone.ι
          (mapCocone (lightProfiniteToLightCondSet ⋙ free R)
            (coproductCocone (P := fun (X : TopCat.{0}) ↦
              TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
              (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)))))
        { as := WalkingPair.left })
      (CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) (cover π) ≫
        CategoryTheory.Functor.map
            (parallelPair (X :=
                lightProfiniteToLightCondSet.obj
                  (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
                (lightProfiniteToLightCondSet.map
                  (CompHausLike.pullback.fst (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
                (lightProfiniteToLightCondSet.map
                  (CompHausLike.pullback.snd (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π)) ⋙
              free R)
            WalkingParallelPairHom.right ≫
          (g -
              CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R)
                  (r_inf ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞})) ≫
                g +
            CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R)
                (r_inf ≫
                  LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫
                    π ≫ fst S ℕ∪{∞} ≫ σ ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞})) ≫
              g)))
have left : HAdd.hAdd (β := obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
    (HSub.hSub (α := obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X) (β :=
      obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
      (CategoryTheory.Functor.map (free R)
            (CategoryStruct.comp (Y :=
              lightProfiniteToLightCondSet.obj
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
              (lightProfiniteToLightCondSet.map
                (NatTrans.app
                    (Cocone.ι
                      (coproductCocone (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                        (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                          LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                    { as := WalkingPair.left : Discrete WalkingPair } ≫
                  cover π))
              (lightProfiniteToLightCondSet.map
                (CompHausLike.pullback.fst (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))) ≫
          g :
        obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
      (CategoryTheory.Functor.map (free R)
            (lightProfiniteToLightCondSet.map
                (NatTrans.app
                    (Cocone.ι
                      (coproductCocone (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                        (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                          LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                    { as := WalkingPair.left : Discrete WalkingPair } ≫
                  cover π ≫
                    CompHausLike.pullback.fst (P := fun (X : TopCat.{0}) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π) ≫
              lightProfiniteToLightCondSet.map (r_inf ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}))) ≫
          g :
        obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X))
    (CategoryTheory.Functor.map (free R)
          (lightProfiniteToLightCondSet.map
              (NatTrans.app
                  (Cocone.ι
                    (coproductCocone (P := fun (X : TopCat.{0}) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                      (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                        LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                        (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                        (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                  { as := WalkingPair.left : Discrete WalkingPair } ≫
                cover π ≫
                  CompHausLike.pullback.fst (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π) ≫
            lightProfiniteToLightCondSet.map
              (r_inf ≫
                LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫
                  π ≫ fst S ℕ∪{∞} ≫ σ ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}))) ≫
        g :
      obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X) =
  HAdd.hAdd (β := obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
    (HSub.hSub (α := obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X) (β :=
      obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
      (CategoryTheory.Functor.map (free R)
            (CategoryStruct.comp (Y :=
              lightProfiniteToLightCondSet.obj
                (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))
              (lightProfiniteToLightCondSet.map
                (NatTrans.app
                    (Cocone.ι
                      (coproductCocone (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                        (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                          LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                    { as := WalkingPair.left : Discrete WalkingPair } ≫
                  cover π))
              (lightProfiniteToLightCondSet.map
                (CompHausLike.pullback.snd (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π))) ≫
          g :
        obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
      (CategoryTheory.Functor.map (free R)
            (lightProfiniteToLightCondSet.map
                (NatTrans.app
                    (Cocone.ι
                      (coproductCocone (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                        (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                          LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                          (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                    { as := WalkingPair.left : Discrete WalkingPair } ≫
                  cover π ≫
                    CompHausLike.pullback.snd (P := fun (X : TopCat.{0}) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π) ≫
              lightProfiniteToLightCondSet.map (r_inf ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}))) ≫
          g :
        obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X))
    (CategoryTheory.Functor.map (free R)
          (lightProfiniteToLightCondSet.map
              (NatTrans.app
                  (Cocone.ι
                    (coproductCocone (P := fun (X : TopCat.{0}) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) T
                      (CompHausLike.pullback (P := fun (X : TopCat.{0}) ↦
                        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) (X :=
                        LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞})) (Y := LightProfinite.fibre ∞ (π ≫ snd S ℕ∪{∞}))
                        (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π)
                        (LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫ π))))
                  { as := WalkingPair.left } ≫
                cover π ≫
                  CompHausLike.pullback.snd (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) π π) ≫
            lightProfiniteToLightCondSet.map
              (r_inf ≫
                LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}) ≫
                  π ≫ fst S ℕ∪{∞} ≫ σ ≫ LightProfinite.fibreIncl ∞ (π ≫ snd S ℕ∪{∞}))) ≫
        g :
      obj (free R) (lightProfiniteToLightCondSet.obj T) ⟶ X)
Tactic: simp [← map_comp_assoc, -Functor.map_comp]