∀ {α : Type u} {r : Std.Roi α} [inst : LT α] [inst_1 : DecidableLT α] [inst_2 : Std.PRange.UpwardEnumerable α]
  [inst_3 : Std.PRange.LawfulUpwardEnumerable α] [inst_4 : Std.PRange.LawfulUpwardEnumerableLT α]
  [inst_5 : Std.Rxi.IsAlwaysFinite α] {pref suff : Array α} {cur : α} (h : r.toArray = pref ++ #[cur] ++ suff),
  cur = (Std.PRange.succMany? (pref.size + 1) r.lower).get ⋯