-- Lean 4 proof step from Mathlib.Algebra.Category.ModuleCat.Sheaf.PullbackFree (hypothesis)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
D : Type u₂
inst_1 : Category.{v₂, u₂} D
J : GrothendieckTopology.{v₁, u₁} C
K : GrothendieckTopology.{v₂, u₂} D
F : CategoryTheory.Functor.{v₁, v₂, u₁, u₂} C D
S : Sheaf J RingCat
R : Sheaf K RingCat
inst_2 : F.IsContinuous J K
φ : S ⟶ Functor.obj (F.sheafPushforwardContinuous RingCat J K) R
inst_3 :
  J.HasSheafCompose
    (forget₂ RingCat (FC := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := RingCat.carrier)
      AddCommGrpCat (FD := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CD :=
      AddCommGrpCat.carrier))
inst_4 :
  K.HasSheafCompose
    (forget₂ RingCat (FC := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := RingCat.carrier)
      AddCommGrpCat (FD := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CD :=
      AddCommGrpCat.carrier))
M : SheafOfModules R
f : unit R ⟶ M
X : Cᵒᵖ
⊢ Eq (α :=
    (((pushforward φ).obj M).val.presheaf ⋙
          CategoryTheory.forget Ab (FC := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CC :=
            AddCommGrpCat.carrier)).obj
      X)
    ((↑(pushforwardSections φ ((M.unitHomEquiv : (a : unit R ⟶ M) → M.sections) f)) :
          (j : Cᵒᵖ) → (((pushforward φ).obj M).val.presheaf ⋙ CategoryTheory.forget Ab).obj j)
        X :
      (((pushforward φ).obj M).val.presheaf ⋙
            CategoryTheory.forget Ab (FC := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CC :=
              AddCommGrpCat.carrier)).obj
        X)
    ((↑((((pushforward φ).obj M).unitHomEquiv : (a : unit S ⟶ (pushforward φ).obj M) → ((pushforward φ).obj M).sections)
              (unitToPushforwardObjUnit φ ≫ (pushforward φ).map f)) :
          (j : Cᵒᵖ) → (((pushforward φ).obj M).val.presheaf ⋙ CategoryTheory.forget Ab).obj j)
        X :
      (((pushforward φ).obj M).val.presheaf ⋙
            CategoryTheory.forget Ab (FC := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CC :=
              AddCommGrpCat.carrier)).obj
        X)
have this : Eq (α := (↑(((pushforward φ).obj (unit R)).val.obj X) : Type u))
  ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat (↑(S.obj (P := Presheaf.IsSheaf J).obj X) : Type u)) ↦
          LinearMap (R := (↑(S.obj (P := Presheaf.IsSheaf J).obj X) : Type u))
            (RingHom.id (↑(S.obj (P := Presheaf.IsSheaf J).obj X) : Type u)) (↑x1 : Type u) (↑x2 : Type u))
          (CC := ModuleCat.carrier) (X := (unit S).val.obj X) (Y := ((pushforward φ).obj (unit R)).val.obj X)
          (PresheafOfModules.Hom.app (Hom.val (unitToPushforwardObjUnit φ)) X) :
        (a : (↑((unit S).val.obj X) : Type u)) → (↑(((pushforward φ).obj (unit R)).val.obj X) : Type u))
      1 :
    (↑(((pushforward φ).obj (unit R)).val.obj X) : Type u))
  ((ConcreteCategory.hom (FC := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := RingCat.carrier)
          (φ.hom.app X) :
        (a : (↑(S.obj.obj X) : Type u)) → (↑(((F.sheafPushforwardContinuous RingCat J K).obj R).obj.obj X) : Type u))
      1 :
    (↑((Functor.obj (F.sheafPushforwardContinuous RingCat J K) R).obj (P := Presheaf.IsSheaf J).obj X) : Type u))
Tactic: have := unitToPushforwardObjUnit_val_app_apply φ (X := X) 1