-- Lean 4 proof step from Mathlib.AlgebraicGeometry.Cover.Over (goal)
Goal:
P : MorphismProperty Scheme
S : Scheme
inst : P.IsStableUnderBaseChange
inst_1 : IsJointlySurjectivePreserving P
X W : Scheme
𝒰 : Cover.{u_1, u} (precoverage P) X
f : W ⟶ X
inst_2 : W.Over S
inst_3 : X.Over S
inst_4 : Cover.Over S (P := P) 𝒰
inst_5 : Hom.IsOver f S
Q : MorphismProperty Scheme
inst_6 : Q.HasOfPostcompProperty Q
inst_7 : Q.IsStableUnderBaseChange
inst_8 : Q.IsStableUnderComposition
hX : Q (X ↘ S)
hW : Q (W ↘ S)
hQ : ∀ (j : 𝒰.I₀), Q (𝒰.X j ↘ S)
x : ↥W
⊢ ∃ (i :
    PreZeroHypercover.I₀.{u_1, u, u + 1} (C := Scheme) (S := W)
      { I₀ := 𝒰.I₀,
        X := fun (x : 𝒰.I₀) ↦
          (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
              (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S ⋯) (Z :=
                X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
        f := fun (x : 𝒰.I₀) ↦
          CommaMorphism.left
            (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
              (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) }),
    Membership.mem (γ := Set ↥W)
      (Set.range (ι :=
        ↥(PreZeroHypercover.X
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S ⋯) (Z :=
                      X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S)
                      (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
              f := fun (x : 𝒰.I₀) ↦
                CommaMorphism.left
                  (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
        ⇑(PreZeroHypercover.f
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S ⋯) (Z :=
                      X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S)
                      (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
              f := fun (x : 𝒰.I₀) ↦
                CommaMorphism.left
                  (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
      x
have refine_1 : ∀ (i : (Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀)
  (hy :
    ∃ (y : ↥((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i)), ((Precoverage.ZeroHypercover.pullback₁ f 𝒰).f i) y = x),
  ∃ (i :
    PreZeroHypercover.I₀.{u_1, u, u + 1} (C := Scheme) (S := W)
      { I₀ := 𝒰.I₀,
        X := fun (x : 𝒰.I₀) ↦
          (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
              (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S sorry) (Z :=
                X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
        f := fun (x : 𝒰.I₀) ↦
          CommaMorphism.left
            (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
              (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) }),
    Membership.mem (γ := Set ↥W)
      (Set.range (ι :=
        ↥(PreZeroHypercover.X
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S sorry) (Z :=
                      X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S)
                      (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
              f := fun (x : 𝒰.I₀) ↦
                CommaMorphism.left
                  (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
        ⇑(PreZeroHypercover.f
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (MorphismProperty.Comma.toComma (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback (X := W.asOverProp (P := Q) S hW) (Y := (𝒰.X x).asOverProp (P := Q) S sorry) (Z :=
                      X.asOverProp (P := Q) S hX) (Hom.asOverProp (P := Q) f S)
                      (Hom.asOverProp (P := Q) (𝒰.f x) S))).left,
              f := fun (x : 𝒰.I₀) ↦
                CommaMorphism.left
                  (MorphismProperty.Comma.Hom.toCommaMorphism (P := Q) (Q := ⊤) (W := ⊤)
                    (pullback.fst (Hom.asOverProp (P := Q) f S) (Hom.asOverProp (P := Q) (𝒰.f x) S))) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
      x
Tactic: obtain ⟨i, hy⟩ := Cover.exists_eq (𝒰.pullback₁ f) x