apply Fin.eq_of_val_eq
α : Type u_1
inst✝ : BEq α
xs : List α
x : α
xxs : DifferentList (x :: xs)
i j : Fin (List.length xs)
iej : List.get xs i = List.get xs j
⊢ i = j
case a
α : Type u_1
inst✝ : BEq α
xs : List α
x : α
xxs : DifferentList (x :: xs)
i j : Fin (List.length xs)
iej : List.get xs i = List.get xs j
⊢ i.val = j.val