apply card_sdiff_insert 
  (uniq_part' hvi.1 hj.1 hj.2.symm hvi.2)
    (mem_part hvi.1 hvi.2)
case neg
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 \ insert v (MultiPart.P M j)) = card (M.A \ MultiPart.P M j) - 1
no goals