-- Lean 4 proof step from Mathlib.AlgebraicGeometry.ColimitsOver (goal)
Goal:
P : MorphismProperty Scheme
inst : P.IsStableUnderBaseChange
inst_1 : P.IsMultiplicative
S : Scheme
J : Type u_1
inst_2 : Category.{v_1, u_1} J
D : CategoryTheory.Functor.{v_1, u, u_1, u + 1} J (P.Over ⊤ S)
𝒰 : OpenCover.{u_2, u} S
inst_3 : Category.{v_2, u_2} 𝒰.I₀
inst_4 : LocallyDirected.{v_2, u, u_2} (P := IsOpenImmersion) 𝒰
d : ColimitGluingData.{v_2, u, v_1, u_1, u_2} (P := P) D 𝒰
inst_5 :
  ∀ {i j : 𝒰.I₀} (hij : i ⟶ j),
    PreservesColimitsOfShape.{v_1, u_1, u, u, u + 1, u + 1} (C := P.Over ⊤ (𝒰.X j)) J
      (MorphismProperty.Over.pullback P ⊤ (Cover.trans (P := IsOpenImmersion) 𝒰 hij))
inst_6 : Quiver.IsThin.{u_2, v_2} 𝒰.I₀
inst_7 : Small.{u, u_2} 𝒰.I₀
inst_8 : IsZariskiLocalAtTarget P
𝒱 : OpenCover.{u_2, u} d.glued (P := P).toComma (P := P) (Q := ⊤) (W := ⊤).left := d.relativeGluingData (P := P).cover
s : Cocone D
a : J
𝒲 : (a : J) → OpenCover.{u_2, u} (D.obj a).toComma (P := P) (Q := ⊤) (W := ⊤).left :=
  fun (a : J) ↦ Precoverage.ZeroHypercover.pullback₁ (D.obj a).toComma (P := P) (Q := ⊤) (W := ⊤).hom 𝒰
i : (𝒲 a).I₀
⊢ ((d.cocone (P := P) i).ι.app a).toCommaMorphism (P := P) (Q := ⊤) (W := ⊤).left ≫
      𝒱.f i ≫
        CommaMorphism.left
          (OpenCover.glueMorphismsOverOfLocallyDirected.{v_2, u, u_2} 𝒱
            (fun (i : 𝒱.I₀) ↦
              CategoryStruct.comp (Y := pullback s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i))
                (CommaMorphism.left
                    (Comma.Hom.toCommaMorphism (P := P) (Q := ⊤) (W := ⊤)
                      ((d.isColimit (P := P) i).desc
                        (Functor.mapCocone (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) s))) :
                  (d.cocone (P := P) i).pt.toComma (P := P) (Q := ⊤) (W := ⊤).left ⟶
                    (Functor.obj (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) s.pt).toComma (P := P) (Q := ⊤) (W :=
                        ⊤).left)
                (pullback.fst s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) :
                  pullback s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) ⟶
                    s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).left))
            ⋯ ⋯) =
    pullback.fst (D.obj a).toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) ≫
      (s.ι.app a).toCommaMorphism (P := P) (Q := ⊤) (W := ⊤).left
have refine_2 : CategoryStruct.comp (Y :=
    Over.left ((Functor.obj (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) s.pt).toComma (P := P) (Q := ⊤) (W := ⊤)))
    (Over.Hom.left
        (Comma.Hom.hom (P := P) (Q := ⊤) (W := ⊤)
          ((d.cocone (P := P) i).ι.app a ≫
            (d.isColimit (P := P) i).desc (Functor.mapCocone (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) s))) :
      Over.left
          ((Functor.obj (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) (D.obj a)).toComma (P := P) (Q := ⊤) (W := ⊤)) ⟶
        Over.left ((Functor.obj (MorphismProperty.Over.pullback P ⊤ (𝒰.f i)) s.pt).toComma (P := P) (Q := ⊤) (W := ⊤)))
    (pullback.fst s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) :
      pullback s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) ⟶ s.pt.toComma (P := P) (Q := ⊤) (W := ⊤).left) =
  pullback.fst (D.obj a).toComma (P := P) (Q := ⊤) (W := ⊤).hom (𝒰.f i) ≫
    (s.ι.app a).toCommaMorphism (P := P) (Q := ⊤) (W := ⊤).left
Tactic: rw [Scheme.OpenCover.map_glueMorphismsOverOfLocallyDirected_left, ← Over.comp_left_assoc,
      ← Comma.comp_hom]