∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF → ∀ {k : α}, (∀ {km : α}, t.minKey? = some km → ¬cmp k km = Ordering.eq) → (t.erase k).minKey? = t.minKey?