rwa [sdiff_union_self_eq_union, left_eq_union_iff_subset]
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
M : MultiPart α
hi : i ∈ range (M.t + 1)
this : MultiPart.P M i ⊆ M.A
⊢ M.A = M.A \ MultiPart.P M i ∪ MultiPart.P M i
no goals