-- 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
⊢ (∀ (x : ↥W),
      ∃ (i :
        PreZeroHypercover.I₀.{u_1, u, u + 1} (C := Scheme) (S := W)
          { I₀ := 𝒰.I₀,
            X := fun (x : 𝒰.I₀) ↦
              (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                  (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
            f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) }),
        Membership.mem (γ := Set ↥W)
          (Set.range (ι :=
            ↥(PreZeroHypercover.X
                { I₀ := 𝒰.I₀,
                  X := fun (x : 𝒰.I₀) ↦
                    (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                        (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
                  f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) :
                  PreZeroHypercover.{u_1, u, u + 1} W }
                i))
            ⇑(PreZeroHypercover.f
                { I₀ := 𝒰.I₀,
                  X := fun (x : 𝒰.I₀) ↦
                    (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                        (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
                  f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) :
                  PreZeroHypercover.{u_1, u, u + 1} W }
                i))
          x) ∧
    ∀
      (i :
        PreZeroHypercover.I₀.{u_1, u, u + 1} (C := Scheme) (S := W)
          { I₀ := 𝒰.I₀,
            X := fun (x : 𝒰.I₀) ↦
              (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                  (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
            f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) }),
      P
        (PreZeroHypercover.f
          { I₀ := 𝒰.I₀,
            X := fun (x : 𝒰.I₀) ↦
              (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                  (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
            f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) :
            PreZeroHypercover.{u_1, u, u + 1} W }
          i)
have refine_1 : ∀ (x : ↥W),
  ∃ (i :
    PreZeroHypercover.I₀.{u_1, u, u + 1} (C := Scheme) (S := W)
      { I₀ := 𝒰.I₀,
        X := fun (x : 𝒰.I₀) ↦
          (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
              (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
        f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) }),
    Membership.mem (γ := Set ↥W)
      (Set.range (ι :=
        ↥(PreZeroHypercover.X
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                    (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
              f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
        ⇑(PreZeroHypercover.f
            { I₀ := 𝒰.I₀,
              X := fun (x : 𝒰.I₀) ↦
                (pullback (X := OverClass.asOver W S) (Y := OverClass.asOver (𝒰.X x) S) (Z := OverClass.asOver X S)
                    (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)).left,
              f := fun (x : 𝒰.I₀) ↦ Over.Hom.left (pullback.fst (Hom.asOver f S) (Hom.asOver (𝒰.f x) S)) :
              PreZeroHypercover.{u_1, u, u + 1} W }
            i))
      x
Tactic: refine ⟨fun x ↦ ?_, fun j ↦ ?_⟩