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