-- Lean 4 proof step from Mathlib.Topology.Sheaves.SheafCondition.EqualizerProducts (goal)
Goal:
C : Type u
inst : Category.{v, u} C
inst_1 : HasProducts.{v', v, u} C
X : TopCat
F : Presheaf.{v', v, u} C X
ι : Type v'
U : ι → Opens (↑X : Type v')
c : Cone (SheafConditionEqualizerProducts.diagram F U)
i a : ι
f' : unop (op (single a)) ⟶ unop (op (single i))
⊢ Eq.{v + 1} (α :=
    Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op (single i)) ⟶
      (Functor.op (diagram U) ⋙ F).obj (op (single a)))
    (CategoryTheory.Functor.map (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) f'.op ≫
      Opposite.rec (motive := fun (x : (CategoryTheory.Pairwise ι)ᵒᵖ) ↦
        Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) x ⟶
          (Functor.op (diagram U) ⋙ F).obj x)
        (fun (x : CategoryTheory.Pairwise ι) ↦
          Pairwise.casesOn (motive := fun (x : CategoryTheory.Pairwise ι) ↦
            Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op x) ⟶
              (Functor.op (diagram U) ⋙ F).obj (op x))
            x (fun (i : ι) ↦ c.π.app WalkingParallelPair.zero ≫ Pi.π (fun (i : ι) ↦ F.obj (op (U i))) i) fun (i j : ι) ↦
            c.π.app WalkingParallelPair.one ≫ Pi.π (fun (p : ι × ι) ↦ F.obj (op (U p.1 ⊓ U p.2))) (i, j))
        (op (single a)))
    (Opposite.rec (motive := fun (x : (CategoryTheory.Pairwise ι)ᵒᵖ) ↦
        Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) x ⟶
          (Functor.op (diagram U) ⋙ F).obj x)
        (fun (x : CategoryTheory.Pairwise ι) ↦
          Pairwise.casesOn (motive := fun (x : CategoryTheory.Pairwise ι) ↦
            Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op x) ⟶
              (Functor.op (diagram U) ⋙ F).obj (op x))
            x (fun (i : ι) ↦ c.π.app WalkingParallelPair.zero ≫ Pi.π (fun (i : ι) ↦ F.obj (op (U i))) i) fun (i j : ι) ↦
            c.π.app WalkingParallelPair.one ≫ Pi.π (fun (p : ι × ι) ↦ F.obj (op (U p.1 ⊓ U p.2))) (i, j))
        (op (single i)) ≫
      (Functor.op (diagram U) ⋙ F).map f'.op)
have op : Eq.{v + 1} (α :=
  Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op (single i)) ⟶
    (Functor.op (diagram U) ⋙ F).obj (op (single i)))
  (CategoryStruct.comp (Y :=
    Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op (single i)))
    (CategoryTheory.Functor.map (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt)
      (Quiver.Hom.op (id_single i)))
    (Opposite.rec (motive := fun (x : (CategoryTheory.Pairwise ι)ᵒᵖ) ↦
      Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) x ⟶
        (Functor.op (diagram U) ⋙ F).obj x)
      (fun (x : CategoryTheory.Pairwise ι) ↦
        Pairwise.casesOn (motive := fun (x : CategoryTheory.Pairwise ι) ↦
          Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op x) ⟶
            (Functor.op (diagram U) ⋙ F).obj (op x))
          x (fun (i : ι) ↦ c.π.app WalkingParallelPair.zero ≫ Pi.π (fun (i : ι) ↦ F.obj (op (U i))) i) fun (i j : ι) ↦
          c.π.app WalkingParallelPair.one ≫ Pi.π (fun (p : ι × ι) ↦ F.obj (op (U p.1 ⊓ U p.2))) (i, j))
      (op (single i))))
  (CategoryStruct.comp (Y := (Functor.op (diagram U) ⋙ F).obj (op (single i)))
    (Opposite.rec (motive := fun (x : (CategoryTheory.Pairwise ι)ᵒᵖ) ↦
      Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) x ⟶
        (Functor.op (diagram U) ⋙ F).obj x)
      (fun (x : CategoryTheory.Pairwise ι) ↦
        Pairwise.casesOn (motive := fun (x : CategoryTheory.Pairwise ι) ↦
          Functor.obj.{v', v, v', u} (Functor.obj (Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ) c.pt) (op x) ⟶
            (Functor.op (diagram U) ⋙ F).obj (op x))
          x (fun (i : ι) ↦ c.π.app WalkingParallelPair.zero ≫ Pi.π (fun (i : ι) ↦ F.obj (op (U i))) i) fun (i j : ι) ↦
          c.π.app WalkingParallelPair.one ≫ Pi.π (fun (p : ι × ι) ↦ F.obj (op (U p.1 ⊓ U p.2))) (i, j))
      (op (single i)))
    ((Functor.op (diagram U) ⋙ F).map (Quiver.Hom.op (id_single i))))
Tactic: rcases f' with ⟨⟩
Cleared hypotheses: a, f'