exact card_sdiff_erase (sub_part hvi.1) hvi.2
case pos
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
v : α
j k : ℕ
M : MultiPart α
hvi : i ∈ range (M.t + 1) ∧ v ∈ MultiPart.P M i
hj : j ∈ range (M.t + 1) ∧ j ≠ i
hk : k ∈ range (M.t + 1)
h✝¹ : ¬(k ≠ i ∧ k ≠ j)
h✝ : k = i
⊢ card (M.A \ erase (MultiPart.P M i) v) = card (M.A \ MultiPart.P M i) + 1
no goals