∀ {α : Type u} {cmp : α → α → Ordering} [inst : Std.TransCmp cmp] {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp}
  {k k' : α} {f : β → β} {hc : k' ∈ Std.DTreeMap.Const.modify t k f},
  Std.DTreeMap.Const.get (Std.DTreeMap.Const.modify t k f) k' hc =
    if heq : cmp k k' = Ordering.eq then f (Std.DTreeMap.Const.get t k ⋯) else Std.DTreeMap.Const.get t k' ⋯