-- Lean 4 proof step from Mathlib.Geometry.RingedSpace.OpenImmersion (goal)
Goal:
C : Type u
inst : Category.{v, u} C
inst_1 : HasLimits.{v, u} C
ι : Type v
F : Discrete ι ⥤ SheafedSpace.{u, v, v} C
inst_2 : HasColimit F
j : Discrete ι
x : (↑(↑((F ⋙ forgetToPresheafedSpace).obj j) : TopCat) : Type v)
as : ι
h : { as := as : Discrete ι } ≠ j
U : Opens (↑(↑(F.obj { as := as : Discrete ι }).toPresheafedSpace : TopCat) : Type v)
y : (↑(↑(F.obj { as := as : Discrete ι }).toPresheafedSpace : TopCat) : Type v)
hy : y ∈ (↑U : Set (↑(↑(F.obj { as := as }).toPresheafedSpace : TopCat) : Type v))
eq :
  Eq (α := (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
    ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{v}) ↦ C((↑X : Type v), (↑Y : Type v))) (CC := TopCat.carrier) (X :=
            Functor.obj (forget C) (F.obj { as := as : Discrete ι }))
            (CategoryStruct.comp (Y := Functor.obj (forget C) (colimit F))
              (CategoryTheory.Functor.map (forget C) (colimit.ι F { as := as }))
              (CategoryStruct.comp (Y := colimit (F ⋙ forget C))
                (Iso.hom (preservesColimitIso (forget C) F) :
                  Functor.obj (forget C) (colimit F) ⟶ colimit (F ⋙ forget C))
                (Iso.hom (HasColimit.isoOfNatIso Discrete.natIsoFunctor) ≫
                    (TopCat.sigmaIsoSigma ((F ⋙ forget C).obj ∘ Discrete.mk)).hom :
                  colimit (F ⋙ forget C) ⟶ TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))))) :
          (a : (↑((forget C).obj (F.obj { as := as })) : Type v)) →
            (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
        y :
      (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
    ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{v}) ↦ C((↑X : Type v), (↑Y : Type v))) (CC := TopCat.carrier)
            (CategoryStruct.comp (Y := Functor.obj (forget C) (colimit F))
              (CategoryTheory.Functor.map (forget C) (colimit.ι F j))
              (CategoryStruct.comp (Y := colimit (F ⋙ forget C))
                (Iso.hom (preservesColimitIso (forget C) F) :
                  Functor.obj (forget C) (colimit F) ⟶ colimit (F ⋙ forget C))
                (Iso.hom (HasColimit.isoOfNatIso Discrete.natIsoFunctor) ≫
                    (TopCat.sigmaIsoSigma ((F ⋙ forget C).obj ∘ Discrete.mk)).hom :
                  colimit (F ⋙ forget C) ⟶ TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))))) :
          (a : (↑((forget C).obj (F.obj j)) : Type v)) →
            (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
        x :
      (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
⊢ False
have h₁ : ∀ (as_1 : ι) (x : (↑(↑((F ⋙ forgetToPresheafedSpace).obj { as := as_1 : Discrete ι }) : TopCat) : Type v))
  (h : { as := as : Discrete ι } ≠ { as := as_1 })
  (eq :
    Eq (α := (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
      ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{v}) ↦ C((↑X : Type v), (↑Y : Type v))) (CC := TopCat.carrier)
              (X := Functor.obj (forget C) (F.obj { as := as : Discrete ι }))
              (CategoryStruct.comp (Y := Functor.obj (forget C) (colimit F))
                (CategoryTheory.Functor.map (forget C) (colimit.ι F { as := as }))
                (CategoryStruct.comp (Y := colimit (F ⋙ forget C))
                  (Iso.hom (preservesColimitIso (forget C) F) :
                    Functor.obj (forget C) (colimit F) ⟶ colimit (F ⋙ forget C))
                  (Iso.hom (HasColimit.isoOfNatIso Discrete.natIsoFunctor) ≫
                      (TopCat.sigmaIsoSigma ((F ⋙ forget C).obj ∘ Discrete.mk)).hom :
                    colimit (F ⋙ forget C) ⟶
                      TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))))) :
            (a : (↑((forget C).obj (F.obj { as := as })) : Type v)) →
              (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
          y :
        (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
      ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{v}) ↦ C((↑X : Type v), (↑Y : Type v))) (CC := TopCat.carrier)
              (X := Functor.obj (forget C) (F.obj { as := as_1 : Discrete ι }))
              (CategoryStruct.comp (Y := Functor.obj (forget C) (colimit F))
                (CategoryTheory.Functor.map (forget C) (colimit.ι F { as := as_1 }))
                (CategoryStruct.comp (Y := colimit (F ⋙ forget C))
                  (Iso.hom (preservesColimitIso (forget C) F) :
                    Functor.obj (forget C) (colimit F) ⟶ colimit (F ⋙ forget C))
                  (Iso.hom (HasColimit.isoOfNatIso Discrete.natIsoFunctor) ≫
                      (TopCat.sigmaIsoSigma ((F ⋙ forget C).obj ∘ Discrete.mk)).hom :
                    colimit (F ⋙ forget C) ⟶
                      TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))))) :
            (a : (↑((forget C).obj (F.obj { as := as_1 })) : Type v)) →
              (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))
          x :
        (↑(TopCat.of ((i : ι) × (↑(((F ⋙ forget C).obj ∘ Discrete.mk) i) : Type v))) : Type v))),
  False
Tactic: cases j
Cleared hypotheses: j, x, h, eq