theorem IsPushout.isVanKampen_iff (H : IsPushout f g h i) :
    H.IsVanKampen ↔ IsVanKampenColimit (PushoutCocone.mk h i H.w) := by
  constructor
  · intro H F' c' α fα eα hα
    refine' Iff.trans _
        ((H (F'.map WalkingSpan.Hom.fst) (F'.map WalkingSpan.Hom.snd) (c'.ι.app _) (c'.ι.app _)
          (α.app _) (α.app _) (α.app _) fα (by convert hα WalkingSpan.Hom.fst)
          (by convert hα WalkingSpan.Hom.snd) _ _ _).trans _)
    · have : F'.map WalkingSpan.Hom.fst ≫ c'.ι.app WalkingSpan.left =
          F'.map WalkingSpan.Hom.snd ≫ c'.ι.app WalkingSpan.right := by
        simp only [Cocone.w]
      rw [(IsColimit.equivOfNatIsoOfIso (diagramIsoSpan F') c' (PushoutCocone.mk _ _ this)
            _).nonempty_congr]
      · exact ⟨fun h => ⟨⟨this⟩, h⟩, fun h => h.2⟩
      · refine' Cocones.ext (Iso.refl c'.pt) _
        rintro (_ | _ | _) <;> dsimp <;>
          simp only [c'.w, Category.assoc, Category.id_comp, Category.comp_id]
    · exact ⟨NatTrans.congr_app eα.symm _⟩
    · exact ⟨NatTrans.congr_app eα.symm _⟩
    · exact ⟨by simp⟩
    constructor
    · rintro ⟨h₁, h₂⟩ (_ | _ | _)
      · rw [← c'.w WalkingSpan.Hom.fst]; exact (hα WalkingSpan.Hom.fst).paste_horiz h₁
      exacts [h₁, h₂]
    · intro h; exact ⟨h _, h _⟩
  · introv H W' hf hg hh hi w
    refine'
      Iff.trans _ ((H w.cocone ⟨by rintro (_ | _ | _); exacts [αW, αX, αY], _⟩ αZ _ _).trans _)
    rotate_left
    · rintro i _ (_ | _ | _)
      · dsimp; simp only [Functor.map_id, Category.comp_id, Category.id_comp]
      exacts [hf.w, hg.w]
    · ext (_ | _ | _)
      · dsimp; rw [PushoutCocone.condition_zero]; erw [Category.assoc, hh.w, hf.w_assoc]
      

Proof state:
case mpr.refine'_3.w.h.some.left
J : Type v'
inst✝¹ : Category.{u', v'} J
C : Type u
inst✝ : Category.{v, u} C
W X Y Z : C
f : W ⟶ X
g : W ⟶ Y
h : X ⟶ Z
i : Y ⟶ Z
H✝ : IsPushout f g h i
H : IsVanKampenColimit (PushoutCocone.mk h i (_ : f ≫ h = g ≫ i))
W' X' Y' Z' : C
f' : W' ⟶ X'
g' : W' ⟶ Y'
h' : X' ⟶ Z'
i' : Y' ⟶ Z'
αW : W' ⟶ W
αX : X' ⟶ X
αY : Y' ⟶ Y
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY g
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
⊢ ((NatTrans.mk fun X_1 => Option.casesOn X_1 αW fun val => WalkingPair.casesOn val αX αY) ≫
          (PushoutCocone.mk h i (_ : f ≫ h = g ≫ i)).ι).app
      (some WalkingPair.left) =
    ((CommSq.cocone w).ι ≫ (Functor.const WalkingSpan).map αZ).app (some WalkingPair.left)
case mpr.refine'_3.w.h.some.right
J : Type v'
inst✝¹ : Category.{u', v'} J
C : Type u
inst✝ : Category.{v, u} C
W X Y Z : C
f : W ⟶ X
g : W ⟶ Y
h : X ⟶ Z
i : Y ⟶ Z
H✝ : IsPushout f g h i
H : IsVanKampenColimit (PushoutCocone.mk h i (_ : f ≫ h = g ≫ i))
W' X' Y' Z' : C
f' : W' ⟶ X'
g' : W' ⟶ Y'
h' : X' ⟶ Z'
i' : Y' ⟶ Z'
αW : W' ⟶ W
αX : X' ⟶ X
αY : Y' ⟶ Y
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY g
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
⊢ ((NatTrans.mk fun X_1 => Option.casesOn X_1 αW fun val => WalkingPair.casesOn val αX αY) ≫
          (PushoutCocone.mk h i (_ : f ≫ h = g ≫ i)).ι).app
      (some WalkingPair.right) =
    ((CommSq.cocone w).ι ≫ (Functor.const WalkingSpan).map αZ).app (some WalkingPair.right)

Next tactic:
exacts [hh.w.symm, hi.w.symm]