∀ {α : Type u} [inst : LE α] [inst_1 : DecidableLE α] [inst_2 : Std.PRange.UpwardEnumerable α]
  [inst_3 : Std.PRange.LawfulUpwardEnumerable α] [Std.PRange.LawfulUpwardEnumerableLE α]
  [inst_5 : Std.Rxc.IsAlwaysFinite α] {r : Std.Rcc α} {i : ℕ},
  r.toList[i]? = Option.filter (fun x => decide (x ≤ r.upper)) (Std.PRange.succMany? i r.lower)