-- Mathlib/CategoryTheory/Sites/Sheafification.lean
theorem sheafifyLift_unique {P Q : Cᵒᵖ ⥤ D} (η : P ⟶ Q) (hQ : Presheaf.IsSheaf J Q)
    (γ : sheafify J P ⟶ Q) : toSheafify J P ≫ γ = η → γ = sheafifyLift J η hQ

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ (sheafificationAdjunction J D).unit.app P ≫ (sheafToPresheaf J D).map γ' = η

Tactic:
exact h

Result:
no goals

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
⊢ γ = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Tactic:
let γ' : (presheafToSheaf J D).obj P ⟶ ⟨Q, hQ⟩ := ⟨γ⟩

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ γ = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
⊢ γ = sheafifyLift J η hQ

Tactic:
rw [sheafifyLift]

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
⊢ γ = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
⊢ toSheafify J P ≫ γ = η → γ = sheafifyLift J η hQ

Tactic:
intro h

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : toSheafify J P ≫ γ = η
⊢ γ = sheafifyLift J η hQ

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : toSheafify J P ≫ γ = η
⊢ γ = sheafifyLift J η hQ

Tactic:
rw [toSheafify] at h

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
⊢ γ = sheafifyLift J η hQ

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ γ = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Tactic:
change γ'.val = _

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ γ'.val = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Proof state:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ γ'.val = (((sheafificationAdjunction J D).homEquiv P { val := Q, cond := hQ }).symm η).val

Tactic:
rw [← Sheaf.Hom.ext_iff, ← Adjunction.homEquiv_apply_eq, Adjunction.homEquiv_unit]

Result:
C : Type u₁
inst✝² : Category.{v₁, u₁} C
J : GrothendieckTopology C
D : Type u_1
inst✝¹ : Category.{u_2, u_1} D
inst✝ : HasWeakSheafify J D
P Q : Cᵒᵖ ⥤ D
η : P ⟶ Q
hQ : Presheaf.IsSheaf J Q
γ : sheafify J P ⟶ Q
h : (sheafificationAdjunction J D).unit.app P ≫ γ = η
γ' : (presheafToSheaf J D).obj P ⟶ { val := Q, cond := hQ } := { val := γ }
⊢ (sheafificationAdjunction J D).unit.app P ≫ (sheafToPresheaf J D).map γ' = η