∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t₁ t₂ : Std.ExtDTreeMap α (fun x => β) cmp}
  [inst : Std.TransCmp cmp] {k : α},
  Std.ExtDTreeMap.Const.get? (t₁ ∩ t₂) k = if k ∈ t₂ then Std.ExtDTreeMap.Const.get? t₁ k else none