∀ {α : Type u} {β : α → Type v} [inst : Ord α] [Std.TransOrd α] {k : α}
  {f : Std.DTreeMap.Internal.Cell α β (compare k) → Std.DTreeMap.Internal.Cell α β (compare k)}
  {l : Std.DTreeMap.Internal.Impl α β} (hlb : l.Balanced),
  l.Ordered →
    (Std.DTreeMap.Internal.Impl.updateCell k f l hlb).impl.toListModel =
      List.filter (fun x => compare k x.fst == Ordering.gt) l.toListModel ++
          (f (Std.DTreeMap.Internal.List.findCell l.toListModel (compare k))).inner.toList ++
        List.filter (fun x => compare k x.fst == Ordering.lt) l.toListModel