intro hk
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
v : α
j k : ℕ
M : MultiPart α
hvi : i ∈ range (M.t + 1) ∧ v ∈ MultiPart.P M i
hj : j ∈ range (M.t + 1) ∧ j ≠ i
⊢ k ∈ range (M.t + 1) →
    card (MultiPart.P (move M hvi hj) k) =
      if k ≠ i ∧ k ≠ j then card (MultiPart.P M k)
      else if k = i then card (MultiPart.P M i) - 1 else card (MultiPart.P M j) + 1
α : Type u_1
inst✝¹ : Fintype α
inst✝ : DecidableEq α
i : ℕ
v : α
j k : ℕ
M : MultiPart α
hvi : i ∈ range (M.t + 1) ∧ v ∈ MultiPart.P M i
hj : j ∈ range (M.t + 1) ∧ j ≠ i
hk : k ∈ range (M.t + 1)
⊢ card (MultiPart.P (move M hvi hj) k) =
    if k ≠ i ∧ k ≠ j then card (MultiPart.P M k)
    else if k = i then card (MultiPart.P M i) - 1 else card (MultiPart.P M j) + 1