have : 𝒜 ⊆ {∅} := fun s hs ↦ by rw [mem_singleton, ← Finset.card_eq_zero]; exact h₁.1 hs
case intro.inl
α : Type u_1
inst✝¹ : LinearOrder α
𝒜 𝒜₁ 𝒜₂ : Finset (Finset α)
s t : Finset α
inst✝ : Finite α
val✝ : Fintype α
h₁ : IsInitSeg 𝒜 0
⊢ IsInitSeg (∂ 𝒜) (0 - 1)
case intro.inl
α : Type u_1
inst✝¹ : LinearOrder α
𝒜 𝒜₁ 𝒜₂ : Finset (Finset α)
s t : Finset α
inst✝ : Finite α
val✝ : Fintype α
h₁ : IsInitSeg 𝒜 0
this : 𝒜 ⊆ {∅}
⊢ IsInitSeg (∂ 𝒜) (0 - 1)