∀ {α : Type u_1} {p : α → Bool} (head : α) (tail : List α) {i : ℕ} (s : ℕ)
  (h : i < (List.filter p (head :: tail)).length),
  (List.findIdxs p (head :: tail) s)[i] - s + 1 ≤ (head :: tail).length →
    (List.findIdxs p (head :: tail) s)[i] - s < (head :: tail).length