∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap α β cmp} [Std.TransCmp cmp] [inst : Min α]
  [inst_1 : LE α] [Std.LawfulOrderCmp cmp] [Std.LawfulOrderMin α] [Std.LawfulOrderLeftLeaningMin α]
  [Std.LawfulEqCmp cmp], t.keys.head? = t.minKey?