let ⟨l'',hl''⟩ := restrict_of_not_mem_last this
m n✝ n : Nat
IH : ∀ {l : List (Fin n)}, List.Nodup l → List.length l ≤ n
l : List (Fin (Nat.succ n))
nodup : List.Nodup l
hn : last n ∈ l
l' : List (Fin (Nat.succ n)) := List.erase l (last n)
this✝¹ : List.length l' = Nat.pred (List.length l)
this✝ : List.length l = List.length l' + 1
this : ¬last n ∈ l'
⊢ List.length l' ≤ n
m n✝ n : Nat
IH : ∀ {l : List (Fin n)}, List.Nodup l → List.length l ≤ n
l : List (Fin (Nat.succ n))
nodup : List.Nodup l
hn : last n ∈ l
l' : List (Fin (Nat.succ n)) := List.erase l (last n)
this✝¹ : List.length l' = Nat.pred (List.length l)
this✝ : List.length l = List.length l' + 1
this : ¬last n ∈ l'
l'' : List (Fin n)
hl'' : l' = List.map castSucc l''
⊢ List.length l' ≤ n