∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Std.LawfulEqOrd α],
  t.WF →
    ∀ {k k' fallback : α} {f : Option (β k) → Option (β k)},
      (Std.DTreeMap.Internal.Impl.alter! k f t).getKeyD k' fallback =
        if compare k k' = Ordering.eq then if (f (t.get? k)).isSome = true then k else fallback
        else t.getKeyD k' fallback