∀ {α : Type u_1} {xs : List α} {p : α → Bool} {n : ℕ} {a : α},
  List.findIdxNth p (a :: xs) n =
    if n = 0 then if p a = true then 0 else List.findIdxNth p xs 0 + 1
    else if p a = true then List.findIdxNth p xs (n - 1) + 1 else List.findIdxNth p xs n + 1