∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [inst : BEq α]
  [Std.TransCmp cmp] [Std.LawfulBEqCmp cmp] {k : α} {v : β},
  (Std.DTreeMap.Const.toList (t.insert k v)).Perm
    ((k, v) :: List.filter (fun x => decide ¬(k == x.1) = true) (Std.DTreeMap.Const.toList t))