∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  {k k' fallback : α} {f : Option β → Option β},
  (Std.ExtDTreeMap.Const.alter t k f).getKeyD k' fallback =
    if cmp k k' = Ordering.eq then if (f (Std.ExtDTreeMap.Const.get? t k)).isSome = true then k else fallback
    else t.getKeyD k' fallback