nth_rw 1 [sdiff_part hi]
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
M : MultiPart α
hi : i ∈ range (M.t + 1)
⊢ card M.A = card (M.A \ MultiPart.P M i) + card (MultiPart.P M i)
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
M : MultiPart α
hi : i ∈ range (M.t + 1)
⊢ card (M.A \ MultiPart.P M i ∪ MultiPart.P M i) = card (M.A \ MultiPart.P M i) + card (MultiPart.P M i)