∀ {α : Type u_1} [inst_1 : DecidableEq α] (w w_1 : α)
  (h : List.idxOfNth w_1 [] [].length + 1 ≤ (List.filter (fun x => decide (x = w)) []).length),
  (List.findIdxs (fun x => decide (x = w)) [])[List.idxOfNth w_1 [] [].length] < [].length