exact Nat.succ_pred_eq_of_pos <| List.length_pos_of_mem 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
l' : List (Fin (Nat.succ n)) := List.erase l (last n)
this : List.length l' = Nat.pred (List.length l)
⊢ Nat.pred (List.length l) + 1 = List.length l
no goals