apply Finset.sum_congr rfl _
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
M : MultiPart α
⊢ ∑ x in range (M.t + 1), ∑ i in MultiPart.P M x, degree (mp M) i =
    ∑ i in range (M.t + 1), card (MultiPart.P M i) * card (M.A \ MultiPart.P M i)
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
M : MultiPart α
⊢ ∀ (x : ℕ),
    x ∈ range (M.t + 1) →
      ∑ i in MultiPart.P M x, degree (mp M) i = card (MultiPart.P M x) * card (M.A \ MultiPart.P M x)