∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeSet α cmp} [Std.TransCmp cmp] {k fallback : α},
  (t.erase k).isEmpty = false → ¬cmp k (t.minD fallback) = Ordering.eq → (t.erase k).minD fallback = t.minD fallback