-- Mathlib/CategoryTheory/Sites/CompatiblePlus.lean
theorem plusCompIso_whiskerLeft {F G : D ⥤ E} (η : F ⟶ G) (P : Cᵒᵖ ⥤ D)
    [∀ X : C, PreservesColimitsOfShape (J.Cover X)ᵒᵖ F]
    [∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan F]
    [∀ X : C, PreservesColimitsOfShape (J.Cover X)ᵒᵖ G]
    [∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan G] :
    whiskerLeft _ η ≫ (J.plusCompIso G P).hom =
      (J.plusCompIso F P).hom ≫ J.plusMap (whiskerLeft _ η)

Proof state:
case w.h
C : Type u
inst✝¹⁰ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w₁
inst✝⁹ : Category.{max v u, w₁} D
E : Type w₂
inst✝⁸ : Category.{max v u, w₂} E
inst✝⁷ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) D
inst✝⁶ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) E
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
inst✝⁴ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ E
F G : D ⥤ E
η : F ⟶ G
P : Cᵒᵖ ⥤ D
inst✝³ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ F
inst✝² : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan F
inst✝¹ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ G
inst✝ : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan G
X : Cᵒᵖ
W : (J.Cover (unop X))ᵒᵖ
⊢ (F.mapCocone (colimit.cocone (J.diagram P (unop X)))).ι.app W ≫
      (whiskerLeft (J.plusObj P) η ≫ (J.plusCompIso G P).hom).app X =
    (F.mapCocone (colimit.cocone (J.diagram P (unop X)))).ι.app W ≫
      ((J.plusCompIso F P).hom ≫ J.plusMap (whiskerLeft P η)).app X

Tactic:
dsimp [plusObj, plusMap]

Result:
case w.h
C : Type u
inst✝¹⁰ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w₁
inst✝⁹ : Category.{max v u, w₁} D
E : Type w₂
inst✝⁸ : Category.{max v u, w₂} E
inst✝⁷ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) D
inst✝⁶ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) E
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
inst✝⁴ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ E
F G : D ⥤ E
η : F ⟶ G
P : Cᵒᵖ ⥤ D
inst✝³ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ F
inst✝² : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan F
inst✝¹ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ G
inst✝ : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan G
X : Cᵒᵖ
W : (J.Cover (unop X))ᵒᵖ
⊢ F.map (colimit.ι (J.diagram P (unop X)) W) ≫ η.app (colimit (J.diagram P (unop X))) ≫ (J.plusCompIso G P).hom.app X =
    F.map (colimit.ι (J.diagram P (unop X)) W) ≫
      (J.plusCompIso F P).hom.app X ≫ colimMap (J.diagramNatTrans (whiskerLeft P η) (unop X))

Proof state:
case w.h
C : Type u
inst✝¹⁰ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w₁
inst✝⁹ : Category.{max v u, w₁} D
E : Type w₂
inst✝⁸ : Category.{max v u, w₂} E
inst✝⁷ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) D
inst✝⁶ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) E
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
inst✝⁴ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ E
F G : D ⥤ E
η : F ⟶ G
P : Cᵒᵖ ⥤ D
inst✝³ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ F
inst✝² : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan F
inst✝¹ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ G
inst✝ : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan G
X : Cᵒᵖ
W : (J.Cover (unop X))ᵒᵖ
⊢ F.map (colimit.ι (J.diagram P (unop X)) W) ≫ η.app (colimit (J.diagram P (unop X))) ≫ (J.plusCompIso G P).hom.app X =
    F.map (colimit.ι (J.diagram P (unop X)) W) ≫
      (J.plusCompIso F P).hom.app X ≫ colimMap (J.diagramNatTrans (whiskerLeft P η) (unop X))

Tactic:
simp only [ι_plusCompIso_hom, ι_colimMap, whiskerLeft_app, ι_plusCompIso_hom_assoc,
  NatTrans.naturality_assoc, GrothendieckTopology.diagramNatTrans_app]

Result:
case w.h
C : Type u
inst✝¹⁰ : Category.{v, u} C
J : GrothendieckTopology C
D : Type w₁
inst✝⁹ : Category.{max v u, w₁} D
E : Type w₂
inst✝⁸ : Category.{max v u, w₂} E
inst✝⁷ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) D
inst✝⁶ : ∀ (J : MulticospanShape), HasLimitsOfShape (WalkingMulticospan J) E
inst✝⁵ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
inst✝⁴ : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ E
F G : D ⥤ E
η : F ⟶ G
P : Cᵒᵖ ⥤ D
inst✝³ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ F
inst✝² : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan F
inst✝¹ : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ G
inst✝ : ∀ (X : C) (W : J.Cover X) (P : Cᵒᵖ ⥤ D), PreservesLimit (W.index P).multicospan G
X : Cᵒᵖ
W : (J.Cover (unop X))ᵒᵖ
⊢ η.app (multiequalizer ((unop W).index P)) ≫
      (J.diagramCompIso G P (unop X)).hom.app W ≫ colimit.ι (J.diagram (P ⋙ G) (unop X)) W =
    (J.diagramCompIso F P (unop X)).hom.app W ≫
      Multiequalizer.lift ((unop W).index (P ⋙ G)) ((J.diagram (P ⋙ F) (unop X)).obj W)
          (fun x => Multiequalizer.ι ((unop W).index (P ⋙ F)) x ≫ η.app (P.obj (op x.Y))) ⋯ ≫
        colimit.ι (J.diagram (P ⋙ G) (unop X)) W