∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtTreeSet α cmp} [inst : Std.TransCmp cmp] {k : α},
  t.erase k ≠ ∅ → ∀ {fallback : α}, (cmp (t.minD fallback) ((t.erase k).minD fallback)).isLE = true