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