∀ {α : Type u_1} {x : α} [inst : BEq α] (head : α) (tail : List α) {n s : ℕ}
  {h : n < (List.idxsOf x (head :: tail) s).length} (h_2 : ¬n = 0) (h_3 : (head == x) = true),
  (List.findIdxs (fun x_1 => x_1 == x) tail)[n - 1] < tail.length