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