Module: Mathlib.CategoryTheory.Sites.Hypercover.One

Goal state before:
pp: case refine_2.refine_2
C : Type u
inst✝¹ : Category.{v, u} C
S : C
E : PreOneHypercover S
F : PreOneHypercover S
inst✝ : HasPullbacks C
i j : E.I₀ × F.I₀
W : C
p₁ : W ⟶ pullback (E.f i.1) (F.f i.2)
p₂ : W ⟶ pullback (E.f j.1) (F.f j.2)
w : p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) ≫ E.f i.1 = p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) ≫ E.f j.1
Y : C
p : W ⟶ pullback ((E.inter F).f i) ((E.inter F).f j) := pullback.lift p₁ p₂ w
Z : C
a : Y ⟶ Z
b : Z ⟶ W
k : E.I₁ i.1 j.1
e : Z ⟶ E.Y k
h₁ : b ≫ p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) = e ≫ E.p₁ k
h₂ : b ≫ p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) = e ≫ E.p₂ k
l : F.I₁ i.2 j.2
u : Y ⟶ F.Y l
u₁ : (a ≫ b) ≫ p₁ ≫ pullback.snd (E.f i.1) (F.f i.2) = u ≫ F.p₁ l
u₂ : (a ≫ b) ≫ p₂ ≫ pullback.snd (E.f j.1) (F.f j.2) = u ≫ F.p₂ l
x✝ :
  (Sieve.bind (E.sieve₁ (p₁ ≫ pullback.fst (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.fst (E.f j.1) (F.f j.2))).arrows
        fun x f x_1 ↦
        Sieve.pullback f
          (F.sieve₁ (p₁ ≫ pullback.snd (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.snd (E.f j.1) (F.f j.2)))).arrows
    (a ≫ b)
⊢ (a ≫ b) ≫ p₁ = pullback.lift (a ≫ e) u ⋯ ≫ (E.inter F).p₁ (k, l)

pp: case refine_2.refine_3
C : Type u
inst✝¹ : Category.{v, u} C
S : C
E : PreOneHypercover S
F : PreOneHypercover S
inst✝ : HasPullbacks C
i j : E.I₀ × F.I₀
W : C
p₁ : W ⟶ pullback (E.f i.1) (F.f i.2)
p₂ : W ⟶ pullback (E.f j.1) (F.f j.2)
w : p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) ≫ E.f i.1 = p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) ≫ E.f j.1
Y : C
p : W ⟶ pullback ((E.inter F).f i) ((E.inter F).f j) := pullback.lift p₁ p₂ w
Z : C
a : Y ⟶ Z
b : Z ⟶ W
k : E.I₁ i.1 j.1
e : Z ⟶ E.Y k
h₁ : b ≫ p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) = e ≫ E.p₁ k
h₂ : b ≫ p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) = e ≫ E.p₂ k
l : F.I₁ i.2 j.2
u : Y ⟶ F.Y l
u₁ : (a ≫ b) ≫ p₁ ≫ pullback.snd (E.f i.1) (F.f i.2) = u ≫ F.p₁ l
u₂ : (a ≫ b) ≫ p₂ ≫ pullback.snd (E.f j.1) (F.f j.2) = u ≫ F.p₂ l
x✝ :
  (Sieve.bind (E.sieve₁ (p₁ ≫ pullback.fst (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.fst (E.f j.1) (F.f j.2))).arrows
        fun x f x_1 ↦
        Sieve.pullback f
          (F.sieve₁ (p₁ ≫ pullback.snd (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.snd (E.f j.1) (F.f j.2)))).arrows
    (a ≫ b)
⊢ (a ≫ b) ≫ p₂ = pullback.lift (a ≫ e) u ⋯ ≫ (E.inter F).p₂ (k, l)

Tactic:
· apply pullback.hom_ext
      · simp [h₁]
      · simpa using u₁

Goal state after:
case refine_2.refine_3
C : Type u
inst✝¹ : Category.{v, u} C
S : C
E : PreOneHypercover S
F : PreOneHypercover S
inst✝ : HasPullbacks C
i j : E.I₀ × F.I₀
W : C
p₁ : W ⟶ pullback (E.f i.1) (F.f i.2)
p₂ : W ⟶ pullback (E.f j.1) (F.f j.2)
w : p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) ≫ E.f i.1 = p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) ≫ E.f j.1
Y : C
p : W ⟶ pullback ((E.inter F).f i) ((E.inter F).f j) := pullback.lift p₁ p₂ w
Z : C
a : Y ⟶ Z
b : Z ⟶ W
k : E.I₁ i.1 j.1
e : Z ⟶ E.Y k
h₁ : b ≫ p₁ ≫ pullback.fst (E.f i.1) (F.f i.2) = e ≫ E.p₁ k
h₂ : b ≫ p₂ ≫ pullback.fst (E.f j.1) (F.f j.2) = e ≫ E.p₂ k
l : F.I₁ i.2 j.2
u : Y ⟶ F.Y l
u₁ : (a ≫ b) ≫ p₁ ≫ pullback.snd (E.f i.1) (F.f i.2) = u ≫ F.p₁ l
u₂ : (a ≫ b) ≫ p₂ ≫ pullback.snd (E.f j.1) (F.f j.2) = u ≫ F.p₂ l
x✝ :
  (Sieve.bind (E.sieve₁ (p₁ ≫ pullback.fst (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.fst (E.f j.1) (F.f j.2))).arrows
        fun x f x_1 ↦
        Sieve.pullback f
          (F.sieve₁ (p₁ ≫ pullback.snd (E.f i.1) (F.f i.2)) (p₂ ≫ pullback.snd (E.f j.1) (F.f j.2)))).arrows
    (a ≫ b)
⊢ (a ≫ b) ≫ p₂ = pullback.lift (a ≫ e) u ⋯ ≫ (E.inter F).p₂ (k, l)