∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF →
    ∀ {k k' : α} [inst : Inhabited β] {f : Option β → Option β},
      (t.alter k f)[k']! = if cmp k k' = Ordering.eq then (f t[k]?).get! else t[k']!