rw [card_uni,← sum_erase_add (range (M.t + 1)) _ hj]
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i j : ℕ
M : MultiPart α
hi : i ∈ range (M.t + 1)
hj : j ∈ range (M.t + 1)
hne : i ≠ j
⊢ card (MultiPart.P M i) + card (MultiPart.P M j) ≤ card M.A
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i j : ℕ
M : MultiPart α
hi : i ∈ range (M.t + 1)
hj : j ∈ range (M.t + 1)
hne : i ≠ j
⊢ card (MultiPart.P M i) + card (MultiPart.P M j) ≤
    ∑ x in erase (range (M.t + 1)) j, card (MultiPart.P M x) + card (MultiPart.P M j)