∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  {k : α} [inst_1 : Inhabited β] {f : Option β → Option β},
  Std.ExtDTreeMap.Const.get! (Std.ExtDTreeMap.Const.alter t k f) k = (f (Std.ExtDTreeMap.Const.get? t k)).get!