Module: Mathlib.CategoryTheory.Limits.Constructions.Over.Products

Goal state before:
pp: J : Type w
C : Type u
inst✝ : Category.{v, u} C
X Y Z : C
f : X ⟶ Y
g : X ⟶ Z
⊢ ∀ {X_1 Y_1 : BinaryCofan (Under.mk f) (Under.mk g)} (f_1 : X_1 ⟶ Y_1),
    ({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
                map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯, map_comp := ⋯ } ⋙
              { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
                map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯, map_comp := ⋯ }).map
          f_1 ≫
        (BinaryCofan.ext
            (Under.isoMk
              (Iso.refl
                (({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
                            map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯,
                            map_comp := ⋯ } ⋙
                          { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
                            map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯,
                            map_comp := ⋯ }).obj
                      Y_1).pt.right)
              ⋯)
            ⋯ ⋯).hom =
      (BinaryCofan.ext
            (Under.isoMk
              (Iso.refl
                (({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
                            map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯,
                            map_comp := ⋯ } ⋙
                          { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
                            map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯,
                            map_comp := ⋯ }).obj
                      X_1).pt.right)
              ⋯)
            ⋯ ⋯).hom ≫
        (𝟭 (BinaryCofan (Under.mk f) (Under.mk g))).map f_1

Tactic:
intros

Goal state after:
J : Type w
C : Type u
inst✝ : Category.{v, u} C
X Y Z : C
f : X ⟶ Y
g : X ⟶ Z
X✝ Y✝ : BinaryCofan (Under.mk f) (Under.mk g)
f✝ : X✝ ⟶ Y✝
⊢ ({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
              map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯, map_comp := ⋯ } ⋙
            { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
              map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯, map_comp := ⋯ }).map
        f✝ ≫
      (BinaryCofan.ext
          (Under.isoMk
            (Iso.refl
              (({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
                          map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯,
                          map_comp := ⋯ } ⋙
                        { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
                          map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯,
                          map_comp := ⋯ }).obj
                    Y✝).pt.right)
            ⋯)
          ⋯ ⋯).hom =
    (BinaryCofan.ext
          (Under.isoMk
            (Iso.refl
              (({ obj := fun c ↦ PushoutCocone.mk (Under.Hom.right c.inl) (Under.Hom.right c.inr) ⋯,
                          map := fun {c₁ c₂} a ↦ { hom := Under.Hom.right a.hom, w := ⋯ }, map_id := ⋯,
                          map_comp := ⋯ } ⋙
                        { obj := fun c ↦ BinaryCofan.mk (Under.homMk c.inl ⋯) (Under.homMk c.inr ⋯),
                          map := fun {c₁ c₂} a ↦ { hom := Under.homMk a.hom ⋯, w := ⋯ }, map_id := ⋯,
                          map_comp := ⋯ }).obj
                    X✝).pt.right)
            ⋯)
          ⋯ ⋯).hom ≫
      (𝟭 (BinaryCofan (Under.mk f) (Under.mk g))).map f✝