∀ {α : Type u_1} [inst : BEq α] {xs : List α} {xc : (x : α) × Fin (List.count x xs)},
  (List.findIdxs (fun x => x == xc.fst) xs)[↑xc.snd] + 1 ≤ xs.length →
    (List.findIdxs (fun x => x == xc.fst) xs)[↑xc.snd] < xs.length