intro hfij
case h
α : Type u
β : Type v
l : List α
nodup : Nodup l
f : α → β
hf : ∀ (a₁ a₂ : α), f a₁ = f a₂ → a₁ = a₂
i j : Nat
hi : i < length (List.map f l)
hj : j < length (List.map f l)
⊢ f (get l { val := i, isLt := (_ : { val := i, isLt := hi }.val < length l) }) =
      f (get l { val := j, isLt := (_ : { val := j, isLt := hj }.val < length l) }) →
    i = j
case h
α : Type u
β : Type v
l : List α
nodup : Nodup l
f : α → β
hf : ∀ (a₁ a₂ : α), f a₁ = f a₂ → a₁ = a₂
i j : Nat
hi : i < length (List.map f l)
hj : j < length (List.map f l)
hfij :
  f (get l { val := i, isLt := (_ : { val := i, isLt := hi }.val < length l) }) =
    f (get l { val := j, isLt := (_ : { val := j, isLt := hj }.val < length l) })
⊢ i = j