rw [sum_biUnion (pair_disjoint M)]
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
M : MultiPart α
⊢ ∑ v in Finset.biUnion (range (M.t + 1)) fun i => MultiPart.P M i, degree (mp M) v =
    ∑ 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 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)