-- Mathlib/CategoryTheory/Sites/LeftExact.lean
@[reassoc (attr := simp)]
lemma toSheafify_plusPlusIsoSheafify_hom (P : Cᵒᵖ ⥤ D) :
    J.toSheafify P ≫ (plusPlusIsoSheafify J D P).hom = toSheafify J P

Proof state:
case h.e'_2.h.h.e'_6.h.h.e
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P : Cᵒᵖ ⥤ D
e_1✝ : (P ⟶ sheafify J P) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P))
e_3✝ : P = (𝟭 (Cᵒᵖ ⥤ D)).obj P
e_4✝ : J.sheafify P = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P
⊢ J.toSheafify = (plusPlusAdjunction J D).unit.app

Tactic:
ext1 P

Result:
case h.e'_2.h.h.e'_6.h.h.e.h
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P✝ : Cᵒᵖ ⥤ D
e_1✝ : (P✝ ⟶ sheafify J P✝) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P✝ ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P✝))
e_3✝ : P✝ = (𝟭 (Cᵒᵖ ⥤ D)).obj P✝
e_4✝ : J.sheafify P✝ = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P✝
P : Cᵒᵖ ⥤ D
⊢ J.toSheafify P = (plusPlusAdjunction J D).unit.app P

Proof state:
case h.e'_2.h.h.e'_6.h.h.e.h
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P✝ : Cᵒᵖ ⥤ D
e_1✝ : (P✝ ⟶ sheafify J P✝) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P✝ ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P✝))
e_3✝ : P✝ = (𝟭 (Cᵒᵖ ⥤ D)).obj P✝
e_4✝ : J.sheafify P✝ = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P✝
P : Cᵒᵖ ⥤ D
⊢ J.toPlus P ≫ J.plusMap (J.toPlus P) = (J.toPlus P ≫ J.plusMap (J.toPlus P)) ≫ 𝟙 (J.sheafify P)

Tactic:
rw [Category.comp_id]

Result:
no goals

Proof state:
case h.e'_2.h.h.e'_6.h.h.e.h
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P✝ : Cᵒᵖ ⥤ D
e_1✝ : (P✝ ⟶ sheafify J P✝) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P✝ ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P✝))
e_3✝ : P✝ = (𝟭 (Cᵒᵖ ⥤ D)).obj P✝
e_4✝ : J.sheafify P✝ = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P✝
P : Cᵒᵖ ⥤ D
⊢ J.toSheafify P = (plusPlusAdjunction J D).unit.app P

Tactic:
dsimp [GrothendieckTopology.toSheafify, plusPlusAdjunction]

Result:
case h.e'_2.h.h.e'_6.h.h.e.h
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P✝ : Cᵒᵖ ⥤ D
e_1✝ : (P✝ ⟶ sheafify J P✝) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P✝ ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P✝))
e_3✝ : P✝ = (𝟭 (Cᵒᵖ ⥤ D)).obj P✝
e_4✝ : J.sheafify P✝ = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P✝
P : Cᵒᵖ ⥤ D
⊢ J.toPlus P ≫ J.plusMap (J.toPlus P) = (J.toPlus P ≫ J.plusMap (J.toPlus P)) ≫ 𝟙 (J.sheafify P)

Proof state:
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P : Cᵒᵖ ⥤ D
⊢ J.toSheafify P ≫ (plusPlusIsoSheafify J D P).hom = toSheafify J P

Tactic:
convert Adjunction.unit_leftAdjointUniq_hom_app
  (plusPlusAdjunction J D) (sheafificationAdjunction J D) P

Result:
case h.e'_2.h.h.e'_6.h.h.e
C : Type u
inst✝⁸ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w
inst✝⁷ : Category.{max v u, w} D
inst✝⁶ : ∀ (P : Cᵒᵖ ⥤ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type (max v u)
inst✝⁴ : (X Y : D) → FunLike (FD X Y) (CD X) (CD Y)
inst✝³ : ConcreteCategory D FD
inst✝² : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D)
inst✝¹ : PreservesLimits (forget D)
inst✝ : (forget D).ReflectsIsomorphisms
P : Cᵒᵖ ⥤ D
e_1✝ : (P ⟶ sheafify J P) = ((𝟭 (Cᵒᵖ ⥤ D)).obj P ⟶ (sheafToPresheaf J D).obj ((presheafToSheaf J D).obj P))
e_3✝ : P = (𝟭 (Cᵒᵖ ⥤ D)).obj P
e_4✝ : J.sheafify P = (plusPlusSheaf J D ⋙ sheafToPresheaf J D).obj P
⊢ J.toSheafify = (plusPlusAdjunction J D).unit.app