-- Lean 4 proof step from Mathlib.AlgebraicGeometry.Gluing (goal)
Goal:
X : Scheme
𝒰 : X.OpenCover
i : (gluedCover 𝒰).J
x : ↥((gluedCover 𝒰).U i)
j : (gluedCover 𝒰).J
y : ↥((gluedCover 𝒰).U j)
h : Eq (α := ↥X) ((𝒰.f i) x : ↥X) ((𝒰.f j) y : ↥X)
e : (TopCat.pullbackCone (forgetToTop.map (𝒰.f i)) (forgetToTop.map (𝒰.f j))).pt ≅
  (PullbackCone.mk (W := forgetToTop.obj (pullback (𝒰.f i) (𝒰.f j))) (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j)))
      (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j))) ⋯).pt :=
  (TopCat.pullbackConeIsLimit (forgetToTop.map (𝒰.f i)) (forgetToTop.map (𝒰.f j))).conePointUniqueUpToIso
    (isLimitOfHasPullbackOfPreservesLimit forgetToTop (𝒰.f i) (𝒰.f j))
⊢ Eq (α :=
      ↥((gluedCover 𝒰).U
          (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
            (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))))
      (((gluedCover 𝒰).f
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y)))
          ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC := TopCat.carrier)
                  e.hom :
                (a : (↑(TopCat.pullbackCone (forgetToTop.map (𝒰.f i)) (forgetToTop.map (𝒰.f j))).pt : Type u)) →
                  (↑(PullbackCone.mk (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j)))
                          (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j))) ⋯).pt :
                    Type u))
              (Subtype.mk (p :=
                fun (p : (↑(forgetToTop.obj (𝒰.X i)) : Type u) × (↑(forgetToTop.obj (𝒰.X j)) : Type u)) ↦
                Eq (α := (↑(forgetToTop.obj X) : Type u))
                  ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                          TopCat.carrier) (forgetToTop.map (𝒰.f i)) :
                        (a : (↑(forgetToTop.obj (𝒰.X i)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                      p.1 :
                    (↑(forgetToTop.obj X) : Type u))
                  ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                          TopCat.carrier) (forgetToTop.map (𝒰.f j)) :
                        (a : (↑(forgetToTop.obj (𝒰.X j)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                      p.2 :
                    (↑(forgetToTop.obj X) : Type u)))
                (x, y) h) :
            (↑(PullbackCone.mk (W := forgetToTop.obj (pullback (𝒰.f i) (𝒰.f j)))
                    (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j))) (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j)))
                    ⋯).pt :
              Type u)) :
        ↥((gluedCover 𝒰).U
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))))
      (Sigma.snd (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x) :
        ↥((gluedCover 𝒰).U
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x)))) ∧
    Eq (α :=
      ↥((gluedCover 𝒰).U
          (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
            (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y))))
      (((gluedCover 𝒰).t
              (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
                (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))
              (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
                (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y)) ≫
            (gluedCover 𝒰).f
              (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
                (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y))
              (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
                (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x)))
          ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC := TopCat.carrier)
                  e.hom :
                (a : (↑(TopCat.pullbackCone (forgetToTop.map (𝒰.f i)) (forgetToTop.map (𝒰.f j))).pt : Type u)) →
                  (↑(PullbackCone.mk (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j)))
                          (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j))) ⋯).pt :
                    Type u))
              (Subtype.mk (p :=
                fun (p : (↑(forgetToTop.obj (𝒰.X i)) : Type u) × (↑(forgetToTop.obj (𝒰.X j)) : Type u)) ↦
                Eq (α := (↑(forgetToTop.obj X) : Type u))
                  ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                          TopCat.carrier) (forgetToTop.map (𝒰.f i)) :
                        (a : (↑(forgetToTop.obj (𝒰.X i)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                      p.1 :
                    (↑(forgetToTop.obj X) : Type u))
                  ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                          TopCat.carrier) (forgetToTop.map (𝒰.f j)) :
                        (a : (↑(forgetToTop.obj (𝒰.X j)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                      p.2 :
                    (↑(forgetToTop.obj X) : Type u)))
                (x, y) h) :
            (↑(PullbackCone.mk (W := forgetToTop.obj (pullback (𝒰.f i) (𝒰.f j)))
                    (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j))) (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j)))
                    ⋯).pt :
              Type u)) :
        ↥((gluedCover 𝒰).U
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y))))
      (Sigma.snd (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y) :
        ↥((gluedCover 𝒰).U
            (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
              (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y))))
have h₁ : Eq (α :=
  ↥((gluedCover 𝒰).U
      (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
        (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))))
  (((gluedCover 𝒰).f
        (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))
        (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) j y)))
      ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC := TopCat.carrier)
              e.hom :
            (a : (↑(TopCat.pullbackCone (forgetToTop.map (𝒰.f i)) (forgetToTop.map (𝒰.f j))).pt : Type u)) →
              (↑(PullbackCone.mk (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j)))
                      (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j))) sorry).pt :
                Type u))
          (Subtype.mk (p := fun (p : (↑(forgetToTop.obj (𝒰.X i)) : Type u) × (↑(forgetToTop.obj (𝒰.X j)) : Type u)) ↦
            Eq (α := (↑(forgetToTop.obj X) : Type u))
              ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                      TopCat.carrier) (forgetToTop.map (𝒰.f i)) :
                    (a : (↑(forgetToTop.obj (𝒰.X i)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                  p.1 :
                (↑(forgetToTop.obj X) : Type u))
              ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u}) ↦ C((↑X : Type u), (↑Y : Type u))) (CC :=
                      TopCat.carrier) (forgetToTop.map (𝒰.f j)) :
                    (a : (↑(forgetToTop.obj (𝒰.X j)) : Type u)) → (↑(forgetToTop.obj X) : Type u))
                  p.2 :
                (↑(forgetToTop.obj X) : Type u)))
            (x, y) h) :
        (↑(PullbackCone.mk (W := forgetToTop.obj (pullback (𝒰.f i) (𝒰.f j)))
                (forgetToTop.map (pullback.fst (𝒰.f i) (𝒰.f j))) (forgetToTop.map (pullback.snd (𝒰.f i) (𝒰.f j)))
                sorry).pt :
          Type u)) :
    ↥((gluedCover 𝒰).U
        (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))))
  (Sigma.snd (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
      (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x) :
    ↥((gluedCover 𝒰).U
        (Sigma.fst (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i))
          (Sigma.mk (β := fun (i : (gluedCover 𝒰).J) ↦ ↥((gluedCover 𝒰).U i)) i x))))
Tactic: constructor