-- Mathlib/CategoryTheory/Adhesive.lean
theorem IsPushout.isVanKampen_inl {W E X Z : C} (c : BinaryCofan W E) [FinitaryExtensive C]
    [HasPullbacks C] (hc : IsColimit c) (f : W ⟶ X) (h : X ⟶ Z) (i : c.pt ⟶ Z)
    (H : IsPushout f c.inl h i) : H.IsVanKampen

Proof state:
case intro.intro.refine_2.mp.intro.intro
C : Type u
inst✝² : Category.{v, u} C
W E X Z : C
c : BinaryCofan W E
inst✝¹ : FinitaryExtensive C
inst✝ : HasPullbacks C
hc : IsColimit c
f : W ⟶ X
h : X ⟶ Z
i : c.pt ⟶ Z
H : IsPushout f c.inl h i
hc₁ : IsColimit (BinaryCofan.mk (c.inr ≫ i) h)
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' ⟶ ((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left }
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY c.inl
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
hc₂ : IsColimit (BinaryCofan.mk g' (pullback.fst αY c.inr))
hc₃ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inl (pullback.snd αY c.inr) αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inl
hc₄✝ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inr αX αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inr
Y'' : C := pullback αZ i
cmp : Y' ⟶ Y'' := pullback.lift i' αY ⋯
e₁ : (g' ≫ cmp) ≫ pullback.snd αZ i = αW ≫ c.inl
e₂ : (pullback.fst αY c.inr ≫ cmp) ≫ pullback.snd αZ i = pullback.snd αY c.inr ≫ c.inr
hc₄ : IsColimit (BinaryCofan.mk (g' ≫ cmp) (pullback.fst αY c.inr ≫ cmp))
⊢ IsPullback i' αY αZ i

Tactic:
rw [← Category.id_comp αZ, ← show cmp ≫ pullback.snd _ _ = αY from pullback.lift_snd _ _ _]

Result:
case intro.intro.refine_2.mp.intro.intro
C : Type u
inst✝² : Category.{v, u} C
W E X Z : C
c : BinaryCofan W E
inst✝¹ : FinitaryExtensive C
inst✝ : HasPullbacks C
hc : IsColimit c
f : W ⟶ X
h : X ⟶ Z
i : c.pt ⟶ Z
H : IsPushout f c.inl h i
hc₁ : IsColimit (BinaryCofan.mk (c.inr ≫ i) h)
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' ⟶ ((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left }
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY c.inl
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
hc₂ : IsColimit (BinaryCofan.mk g' (pullback.fst αY c.inr))
hc₃ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inl (pullback.snd αY c.inr) αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inl
hc₄✝ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inr αX αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inr
Y'' : C := pullback αZ i
cmp : Y' ⟶ Y'' := pullback.lift i' αY ⋯
e₁ : (g' ≫ cmp) ≫ pullback.snd αZ i = αW ≫ c.inl
e₂ : (pullback.fst αY c.inr ≫ cmp) ≫ pullback.snd αZ i = pullback.snd αY c.inr ≫ c.inr
hc₄ : IsColimit (BinaryCofan.mk (g' ≫ cmp) (pullback.fst αY c.inr ≫ cmp))
⊢ IsPullback i' (cmp ≫ pullback.snd αZ i) (𝟙 Z' ≫ αZ) i

Proof state:
case intro.intro.refine_2.mp.intro.intro
C : Type u
inst✝² : Category.{v, u} C
W E X Z : C
c : BinaryCofan W E
inst✝¹ : FinitaryExtensive C
inst✝ : HasPullbacks C
hc : IsColimit c
f : W ⟶ X
h : X ⟶ Z
i : c.pt ⟶ Z
H : IsPushout f c.inl h i
hc₁ : IsColimit (BinaryCofan.mk (c.inr ≫ i) h)
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' ⟶ ((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left }
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY c.inl
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
hc₂ : IsColimit (BinaryCofan.mk g' (pullback.fst αY c.inr))
hc₃ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inl (pullback.snd αY c.inr) αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inl
hc₄✝ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inr αX αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inr
Y'' : C := pullback αZ i
cmp : Y' ⟶ Y'' := pullback.lift i' αY ⋯
e₁ : (g' ≫ cmp) ≫ pullback.snd αZ i = αW ≫ c.inl
e₂ : (pullback.fst αY c.inr ≫ cmp) ≫ pullback.snd αZ i = pullback.snd αY c.inr ≫ c.inr
hc₄ : IsColimit (BinaryCofan.mk (g' ≫ cmp) (pullback.fst αY c.inr ≫ cmp))
⊢ IsPullback i' (cmp ≫ pullback.snd αZ i) (𝟙 Z' ≫ αZ) i

Tactic:
apply IsPullback.paste_vert _ (IsPullback.of_hasPullback αZ i)

Result:
C : Type u
inst✝² : Category.{v, u} C
W E X Z : C
c : BinaryCofan W E
inst✝¹ : FinitaryExtensive C
inst✝ : HasPullbacks C
hc : IsColimit c
f : W ⟶ X
h : X ⟶ Z
i : c.pt ⟶ Z
H : IsPushout f c.inl h i
hc₁ : IsColimit (BinaryCofan.mk (c.inr ≫ i) h)
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' ⟶ ((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left }
αZ : Z' ⟶ Z
hf : IsPullback f' αW αX f
hg : IsPullback g' αW αY c.inl
hh : CommSq h' αX αZ h
hi : CommSq i' αY αZ i
w : CommSq f' g' h' i'
hc₂ : IsColimit (BinaryCofan.mk g' (pullback.fst αY c.inr))
hc₃ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inl (pullback.snd αY c.inr) αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inl
hc₄✝ :
  IsPullback (BinaryCofan.mk ((BinaryCofan.mk g' (pullback.fst αY c.inr)).inr ≫ i') h').inr αX αZ
    (BinaryCofan.mk (c.inr ≫ i) h).inr
Y'' : C := pullback αZ i
cmp : Y' ⟶ Y'' := pullback.lift i' αY ⋯
e₁ : (g' ≫ cmp) ≫ pullback.snd αZ i = αW ≫ c.inl
e₂ : (pullback.fst αY c.inr ≫ cmp) ≫ pullback.snd αZ i = pullback.snd αY c.inr ≫ c.inr
hc₄ : IsColimit (BinaryCofan.mk (g' ≫ cmp) (pullback.fst αY c.inr ≫ cmp))
⊢ IsPullback i' cmp (𝟙 Z') (pullback.fst αZ i)