∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {k k' : α} {f : β k → β k},
  (t.modify k f).get? k' = if h : cmp k k' = Ordering.eq then cast ⋯ (Option.map f (t.get? k)) else t.get? k'