∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {a k : α} {v : β k},
  (t.insert k v).get? a = if h : cmp k a = Ordering.eq then some (cast ⋯ v) else t.get? a