∀ {α : Type u_1} {p : α → Bool} (head : α) (tail : List α) {n : ℕ}
  {h : List.findIdxNth p (head :: tail) n < (head :: tail).length},
  ¬n = 0 → p head = true → List.findIdxNth p tail (n - 1) < tail.length