have : l = [] := by cases l; rfl; exact Fin.elim0 ‹_›
case zero
m n : Nat
l : List (Fin Nat.zero)
nodup : List.Nodup l
⊢ List.length l ≤ Nat.zero
case zero
m n : Nat
l : List (Fin Nat.zero)
nodup : List.Nodup l
this : l = []
⊢ List.length l ≤ Nat.zero