suffices ∂^[i] 𝒞 = powersetCard (r - i) range'k by
  rw [this, card_powersetCard, card_attachFin, card_range]
α : Type u_1
inst✝ : LinearOrder α
s U V : Finset α
n r k i : ℕ
𝒜 𝒞✝ : Finset (Finset (Fin n))
hir : i ≤ r
hrk : r ≤ k
hkn : k ≤ n
h₁ : Set.Sized r ↑𝒜
h₂ : k.choose r ≤ 𝒜.card
range'k : Finset (Fin n) := (range k).attachFin ⋯
𝒞 : Finset (Finset (Fin n)) := powersetCard r range'k
Ccard : 𝒞.card = k.choose r
this : Set.Sized r ↑𝒞
⊢ (∂ ^[i] 𝒞).card = k.choose (r - i)
α : Type u_1
inst✝ : LinearOrder α
s U V : Finset α
n r k i : ℕ
𝒜 𝒞✝ : Finset (Finset (Fin n))
hir : i ≤ r
hrk : r ≤ k
hkn : k ≤ n
h₁ : Set.Sized r ↑𝒜
h₂ : k.choose r ≤ 𝒜.card
range'k : Finset (Fin n) := (range k).attachFin ⋯
𝒞 : Finset (Finset (Fin n)) := powersetCard r range'k
Ccard : 𝒞.card = k.choose r
this : Set.Sized r ↑𝒞
⊢ ∂ ^[i] 𝒞 = powersetCard (r - i) range'k