∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  {k : α} {f : Option β → Option β},
  Std.ExtDTreeMap.Const.alter t k f = ∅ ↔ (t = ∅ ∨ t.size = 1 ∧ k ∈ t) ∧ f (Std.ExtDTreeMap.Const.get? t k) = none