let ⟨l',hl'⟩ := restrict_of_not_mem_last hn
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
⊢ List.length l ≤ Nat.succ 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 n)
hl' : l = List.map castSucc l'
⊢ List.length l ≤ Nat.succ n