∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp]
  [inst : Inhabited β], t₁.WF → t₂.WF → ∀ {k : α}, (t₁ ∩ t₂)[k]! = if k ∈ t₂ then t₁[k]! else default